Vassals will always accept provinces given in peace deals (unless outside their coring range) if they would become overextended or if it outside their coring range)
Allies will not always accept provinces in a peace deal (e.g. You cannot change occupation if an ally (or Colonial Nation) occupies the province (however, allies will automatically transfer control of freshly conquered provinces to you unless they have an interest/claim/core in the province). You can only change occupation if the province if occupied by yourself or your vassal/march/junior partner/client state (not Colonial Nation!).
Click the shield and selct the nation which should get the occupation. If you have Art of War DLC you can (before starting the peace 'negotiations') select the occupied province and 'change occupation' (it shows the shield of the current occupier where the recruitment buttons are in an owned province).
