Wrong, plenty of Europeans fought with the Ottomans or are the Balkans suddenly non European?!? Moldovia, Bulgaria, Romania, Hungary, Serbia, Greeks, Bosnians... plenty of European vasalls who had significant impact on affairs in the Ottoman empire themselves.
Those vassals were not part of the Ottoman empire though. They were semi-independent. The direct vassals were the Greece, Albania, Egypt etc. So Ottoman army never recruited Greeks in Byzantine style. They assigned them Ottoman equipments and gave them Ottoman training. A Client state and a direct vassalship are different. Many states didnt bother to change the equipments or trainings of the recruits from the newly conquered lands but also some did.
This problem can be solved very easily in the game.
-Change the equipment of the new recruits -> Done, problem solved.
-Local militia should keep their own cultural equipments.
Or an in-depth cultural conversion could be implemented:
Give player and NPCs to choose the culture of the town they conquer. Upon conquering a town or a castle we should be given 3 options;
1- "Force your culture on the people" in result of this action we should be able to recruit our faction's troops after sometime like 2 weeks ingame or more, you pick.
We should also get relations hit with the notables of the opposite faction and maybe even decrease in population and production as it will create unrest among the citizens.
Also a high rebellion chance should be present.
2- "Make the population slowly adopt your culture" Similar yet slower version of the above, less hit with relations, more time until we recruit desired troops.
3- "Let the inhabitants keep their cultural traits" Nothing changes, and player/npc can continue to recruit the foreign troops if he/she wants to use them as auxiliary troops.
Or another option could be to take a similar approach to the matter by following total war's religion system.
-Building cultural buildings should change the culture slowly.
-Stationing the desired culture's troops should effect the culture of the town/castle after sometime. Especially if they outnumber the locals.
Its all about equipments really.