The reason to plug exploits like vassal-feeding is not to punish people for liking to vassal-feed, it's to prevent people who don't vassal-feed from feeling as though they're playing the game wrong.
For an example, let's say that when you make cores, there's two buttons. One button lets you make a core for ADM cost, and is for regular players. The other is for players who want to WC, and makes a core for no cost. So if you don't want to use the cheesy no-cost button, just don't click it, right? Except that each time you click the button with a cost, you will feel as though you're doing something wrong, and it takes away from the game.
Vassal feeding is superior to paying ADM cost for cores in basically every way. It's the button without a cost to those who know about it.
Did you know that a lot of people don't know about the "seize colony" button? Did you know a lot of people don't understand how "claim throne" works? Should we remove both of those buttons too, since they "work" for people who understand the game but don't work for people who don't understand the game?
Did you know people use Enforce Peace to farm trust so they can diplo-vassalize more people? How is that not a WC button that only matters for people who do WC?
Did you know people use Diplomatic and Expansion groups to get more Diplo Rep? Do you think people who don't understand what Diplo Rep does use those Idea groups much? Are you deleting or changing those Idea groups because people who don't take them are playing the game wrong?
Your game is full of a thousand things that mean you are "doing something wrong" if you don't do them. Your game will always be like that. All you have done is taken out the most visible and whined-about one.
If you wanted to fix your game, you needed to do a thousand other things besides this. You can't just remove AE and vassal feeding and expect everything to be fine. Stop hacking away at this one like Dr Frankenstein.