You cannot. The closest thing you can do (in the latest 5.2 patch) is lend your troops to another player in multiplayer by attatching an army of yours to an army of theirs. They then control both as if they were one single army.
Denmark shouldn't have many problems as a vassal, but Tunisia is a terrible country in a terrible technology group and you have just given them loads of non-cored poor lands which are going to revolt. The AI is terrible at managing money so if you're going to be vassalising North Africa (which isn't a bad idea since the land is so poor you don't realy want it yourself) then you should make sure that the vassals you release actually have lands they have cores on.
Denmark shouldn't have many problems as a vassal, but Tunisia is a terrible country in a terrible technology group and you have just given them loads of non-cored poor lands which are going to revolt. The AI is terrible at managing money so if you're going to be vassalising North Africa (which isn't a bad idea since the land is so poor you don't realy want it yourself) then you should make sure that the vassals you release actually have lands they have cores on.