The Flame in the Flood

Got it Nov 13, 2017. Started endless mode with 40 ash cake, 9 jerky, and 5 jars of clean water. Didn't kill anything, cheevo popped on day 27. Never had to use jerky, just ate ash cake and various plants. Couple tips, you can sit in calm water at docks until you need sleep, then go onto land. Time passes faster on water than land. I got 3% per day on tracker unless I slept through the day change, then it only gave me 2%, which I believe is why it took me 27 not 22 days. Again it seems the actual achievement is for going 20 days without killing animals.
I did delete saves from Xbox and the cloud before starting, seems to be the thing to do from the forums.

This isn't a guide, as the others here already do a well enough job on that, but an exploit I found by chance that helps a ton.

First, craft a torch. Now when you're on foot (not on the raft) use the left trigger and press right to see the quick use menu with the torch in it. Use the torch and you will see the circle above your head that gives you an idea of how much is left before the torch is used up.

With the torch equipped and burning, use the left trigger to bring up the menu again and highlight the torch, but don't press anything yet. Now when the circle empties the torch is extinguished and you lose it from your inventory. At least, that's what usually happens. However, at the moment the percentage on the torch reaches 0, shown by the circle being empty and even more helpful when you HEAR the sound of the torch going out, press A immediately to unequip it.

You should now be able to use the left trigger and once again look in your inventory and you should have the same torch at 0%. This torch will now last forever, no matter how long or how many times you use it. It won't appear lit but it will still have all the effects of a torch.

You're now free to explore any area without fear of attack from wolves or boars and can even use it to move snakes around to where you can use them to help you kill other animals for skins and meat.

Enjoy, and hopefully this helps someone as much as it did for me!

This achievement guide parallels my guide for Paleo Diet. I purchased the game after the update which somewhat "fixed" this achievement.

Without ever deleting my save game, I went through campaign and a 200 day endless mode run and unlocked all achievements except Paleo Diet and Vegetarian. After completing the 200 day run, I started a new endless mode run (without deleting my save file) focused solely on Paleo Diet and got it to unlock.

I then began another new endless mode run without deleting my save file to try for Vegetarian but my achievement progress remained at 0% so I realized that it was necessary to delete my save file under "manage game." Note that you must select delete file "from everywhere" or the local save will update from the cloud and the problem will persist. If you start a new endless mode run and are not seeing your achievement progress change, you did not properly delete "from everywhere."

Here is my guide and some notes regarding its completion.

First thing to note (as BRARROW does above), the achievement description is inaccurate - the achievement is really for going 20 days without killing animals. I believe this is actually much easier to achieve than 40 days without eating any meat.

It is also critical to understand that the game carries over all items placed in Aesop's bag to the start of a new game. (This is actually a setting activated by default in options menu.) Accordingly, if you are going for Vegetarian, you can do one or more short games just to load Aesop's bag with "food" and then intentionally die before beginning your serious attempt at Vegetarian. (This technique can also be used to load Aesop's bag with cooked meat, jerky, etc. when going for Paleo Diet.) I say "food" here and not "plant foods" because I am pretty sure you could load up with meat in prior runs and still get the achievement in a later "vegetarian" run. See my discussion below on this point.

Upon starting your serious attempt at Vegetarian, as long as you have enough food and water and you are not too cold or fatigued, you can get time to run faster by sitting extra minutes in calm water after departing a location before entering the river. This will reduce the distance you travel over the same period of days, which will delay your entrance to colder regions. You cannot kill animals for pelts, so it is very useful to slow your distance over the 20 days. (However, you could consider placing fur clothing, e.g. White Wolf Jacket, in Aesop's bag in the run before to have it available.) I got to 20 days relying only on insulated clothing (from cat tails) and campfires towards the end fend off hypothermia.

Since the real requirement is only not killing animals, you might wonder if it's ok to eat found meat, jerky or especially animals killed by other animals (such as rabbits killed by snakes or wolves and pigs killed by bears). I can absolutely confirm that eating jerky is ok. I collected some jerky from carts along the river and ate it and it did not cause the achievement to fail. I believe this means that it would also be ok to eat animals killed by other animals. I had collected meat from bear kills and cooked it. I had it ready to eat but I ended up getting the achievement before resorting to it so I can't be sure. I was certainly ok to collect it, cook it and carry it. In view of this, I believe the requirement is really limited to just not killing animals during the current run, so you can "eat" animals as long as they were killed in a previous run or killed by another. (It would be great if someone can confirm that eating cooked meat is also ok as I have only confirmed jerky is ok.)

The indicated progress for this achievement appears to correspond to a 40 day period. This means that your progress will rise gradually towards 50% as you get to 20 days. However, since the true requirement is only 20 days without killing an animal, the achievement will pop just a little after hitting 20 days (50%).

Hope this helps.

After I did my 200 mile run and Paleo diet, progress for this achievement stopped tracking. I deleted the saves from my xbox and the cloud. I started a new endless campaign and managed to unlock the achievement soon after. So I can confirm that it does appear (like others have said) that it unlocks after 20 days without killing animals as I personally unlocked this achievement at the start of day 20. I did NOT eat any cooked meat or jerky though. 2 things I can confirm is that you can eat grubs (didn't eat any night crawlers) and that animals can kill other animals. I had a bear kill 3 wolves and a boar and still unlocked the achievement. There is so much conflicting statements about this achievement so I hope this helps a little. Also having a couple of campfires in your bag can come in really clutch, so always have at least one.

Just got this today. I can confirm that the achievement is going for 20 days without killing any animals. As long as you don't kill the animals you will be fine. If your percent is stuck at 0% you need to delete your cloud save by going to manage the game and clicking the saved data. Some additional tips for others:

Finding jerky or earning meat from missions is safe to consume.

Do not worry about being low on everything during times. The game has a weird difficulty set in where if you're low on resources more will spawn in. An example being low on space so a location will give you a pouch or being low on food and being hungry will usually spawn more corn, berries, etc. This is not always guaranteed though.

The percentage will increase by 2-3% every day until it reaches 50%. At 50% you will earn the achievement. If you don't and the percent went to 0% you must delete your save from the cloud and start over because you may have killed an animal.

Getting the storage upgrade is essential in the beginning for obvious reasons. One being you won't be making pouches so this is one of the first upgrades to get.

Starting from scratch with nothing in the dog's backpack is doable. Since you won't be killing animals for furs you will need to buy the stove upgrade for your raft in order to heat yourself up every once in a while.

If you're looting a place and a wolf/wolves chase you, try finding shelter and sleeping in. Doing this may displace the wolves away from you or have them fall asleep somewhere else on the map.

Loot the carts in the river portions. This is a given but finding jerky on them will give you half a day's portion at least. Try to stop everywhere. With a plant diet, you will need to gather a lot of food. Save the ash cakes for desperate days. Eat up the cooked Yucca, garlic, berries, and even cattails when you have a large amount. Do not forget that when it rains you can fill up cans of water for perfectly drinkable water. Have at least 10-15 cans of water. 5 for emergencies for when you're in the wastelands.

For clothing, the best you can do is insulate all your starting clothing and find a hat and gloves to insulate as well. If you get lucky by getting a leather kit and looting a wolf/boar killed by snakes/bears you can make a decent new clothing item. I never risked looting dead animals but I don't believe looting already killed animals voids the achievement.

This unlocked for me after the sun rose on the 24th day. Staying afloat and not docking will pass time faster. If you have the resources you can buy the shelter upgrade in order to pass time a little bit faster at the cost of using up more resources. Towards day 20 I had 10 Ash cakes and 10 water cans. I would sleep in as much as I can because I got impatient.