There is no reason not to go full vitality really.
Provided the DPS does not outgear you, there is never a case where you'll lose agro.
The only way you lose agro is if you make an error while tanking.
The self healing aspect also doesn't offset it either.
30 strength is not the same as 30 vitality in terms of gain.

Second, it is always good to have too much health than too little.
Having "just enough" to tank it is just flawed logic.
If you have more than enough health, that means the healer doesn't need to focus on you often, and has more time to heal you up.
Always a good thing and never bad