Warhammer 40,000: Mechanicus

To side with Videx you need to complete all 11 of his missions before the final mission that ends the game. His final mission is called Prelude to Annihilation. After the final boss you'll get a different ending than usual and the achievement will pop. You can check how far along you are on the View Campaign screen in Missions. If you're not getting any more missions from him then you just need to keep progressing through the campaign to unlock new areas. This achievement runs parallel to Mother of Xenarites for siding with Scaevola and it's probably best to do a separate playthrough for each.