Rock Band 2

In order to unlock the Impossible Vocal Challenge, you must first beat all of the lower tier vocal challenges. YOU DO NOT HAVE TO BE ON VOCALS IN ORDER TO GET THE ACHIEVEMENT. You don't have to be on Expert either. Also, most of the songs on the Impossible Vocal Challenge are fairly easy on instruments, so get some friends if you're worried about failing out.

If you know anyone who has already unlocked the final challenge, join them and have them select it. Playing just the last one unlocks it, so you don't need to actually go through them all from start to finish. Any difficulty and any instrument works as long as someone in your band is on vocals.

If you're actually trying to do this by singing, instead of being a lazy slacker and finding someone else to do it for you, here's a couple of hints:

The first four songs aren't too hard, especially if you're familiar with them beforehand. Remember that you don't have to sing in the same octave as the actual singer in the band does, and you should be fine.

"Tangled Up in Blue" however... Bob Dylan hits NO actual pitches in the song. You can fake it and get three stars, easy, but even with a passing familiarity with the song, you can at least get four or five stars if you follow this one simple rule: mimic his awful/singular vocal delivery. If you start pronouncing the words like he does, suddenly the strangeness in the line starts making sense. Sorta. Just make sure you're following that vocal guideline and that you've heard the song a couple of times and you should be able to four or five-star the song (at least on Hard) easy.

In order to get this achievement you must beat all of the Vocal Challenges in the play a challenge part of world tour. you can do it on any difficulty.

Ironically, you don't actually have to play vocals for this achievement. The easiest way to do it, if you are not the best singer, (though you can play on easy so its really not that bad) is to find somebody who has this challenge unlocked, get them to play vocals, while you play whatever instrument you want. As long as your band gets past the challenge, this achievement is yours.

if you dont have somebody to play with, you can always start from the easiest vocal challenge and unlock them all until you hit the impossible one and play that, as the 'normal' way to get this achievement.

First of all, if you can sing and are having trouble with this, either lower the difficulty or try listening to the songs online beforehand. This really helped me with Tangled Up in Blue especially.

However, if you're reading this solution, you probably aren't the best singer in the world. In this case, I find it's the easiest to actually not try to sing the song at all. Trying usually just ends in failure. Instead, know the basic tune of the song and just kinda groan out the notes. Similar to humming but keep your mouth open and make sure you're loud enough for the mike to pick you up. If you stay quiet enough, it's easy to hear the notes of the song and imitate them.

Finally, I'm not entirely sure if this would work or not but if you could find a vocal track of any of the songs without the instruments, you may be able to play that on your computer and put the mike up to the speaker. Not sure if that would work or not but as long as there aren't any instrumentals, I can't see why not. Or you could always check out the above solutions and just have someone else do it while you play another instrument.