That might be a little confusing to some folks…
It’s not that you are penalized for killing harder things. You get twice the XP that you always do, and the bonus XP is withdrawn from the vitality “bankâ€.
So for example… let’s say that 100% vitality = 100,000 XP
You go out and kill a mob that would normally give you 250 XP without vitality.
You *get* 250 XP from the mob, and the vitality “bank†matches it for an extra 250 XP. You gain a total of 500 XP, half from the mob and half from your stored vitality.
If you kill something that would give 5 XP, you get 10 XP… half from the mob, half from your stored vitality.
The only place this is not true is (or was) Harclave in Splitpaw. There the exchange rate was not even due to people using it as a fast track to the level cap way back in the day. To keep the zone from being exploited too badly, SOE made it so, let’s say, you kill something for 250 XP, you get 250 from the mob and 250 from vitality, but an extra 150 vitality evaporated out of the bank with no gain to your XP.
So except for that one example, no matter what you’re killing, you will still reach the end of your vitality bar with the same total XP gained. You’ll just get there fast on higher yield mobs because, well, you get more XP.
Hopefully that makes it more clear :)