Why do I get less VP if I attack an opponent I’ve already beaten?

If you beat an opponent, the amount of Victory Points you can earn from them in future attacks is reduced.
This usually means that it makes more sense to attack each player only once, although some very strong opponents will still give you a large amount of VP even after you have beaten them once.
Keep an eye how much VP you can win from your opponents, and attack different enemies to maximize your VP gains!