If there weren't "tiers" for stats and instead each point increased your block/damage or whatever, then I would say make a decision based on what you want to have more of. However with the way they designed the game in order to not have us be able to go back to older content and one shot everything like in other games, I would say go all vitality because even if you were all str, you might not even hit the next tier, let alone only get like 1 or 2 % increase.