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Based on the posts of others, it could be that Hungary is at war with someone. If they are, just wait until peace comes and vassalize away.
 
You need more armies, too. As Hungary normally produces tremendous amounts of armies (which usually get attritioned in the Carpates province), you can either increase your military power by producing armies yourself or let the Hungarians reduce their armies in a war ...
 
Yes, as Austrian Emperor one has to be pretty mean and dupy to one´s best friends and allies, the Hungarians, to get them finally vassalized. ;)
In a recent game, obviously the Austrian AI got tired of courting the Hungarians all the time, revoked alliance and marriage and declared war! And they even got away with it, conquering three provinces. Soon after this the Turks dropped by and took the leftovers... :D

Hartmann