I know it doesn't give HP, it's more the fact that is, for example, 5 defence and +2 VIT better than 20HP
That really depends on the situation, even without knowing the exact effect of some stats. Currently there's no well accepted "in a perfect world x is better" that I'm aware of. If you were fighting something where the enemy focuses on magic, or an ability that could one shot you without sufficient HP for instance, HP would always win over VIT and defense.
It'll take a lot of testing to determine the exact relationship between VIT and defense, it's likely one isn't necessarily better than the other, but that they are related in a way that depending on how much of each you have, one or the other is better.