Does he have a Obelisk? If so, are you at least getting some of that 10k merc? If so, then he is not able to exceed the amount needed since he does not have enough merc.

If your vassal has the merc to give you, the game gives it to you automatically. It's on the vassal to supply you with the merc.

in skogark have difirent rules, u can get ap from vassal if he have more than 10lvl obelisk gives and max what can get from vassals is 1000mp, so if you want get 1000 mp then your vassal should be 7890 mp income, it can be any: fountain, camps, hes vassals, rare buildings...