Vit doesn't increase dmg mitigation. 3K more Hp doesn't really make a difference (you just stay alive 2 more seconds in Hl content). If the full Str tank use his cd def correctly, the healer is suppose to be able to deal damage too.
Personnaly as an Astro i always prefered str Tank in my team just for the principe of optimization. And often there was bad str tank and bad vit tank. If the tank doesn't use his CDs, vit or str, we have to babysit him.