For states that were forced vassalized, perhaps its alright
However, my issue is with released vassals.
Can we change released states to a new category of puppet states? Yes, they are vassals I guess, but they are not in the same league. After all, the game mechanics encourage you to release states. I know the religion they adopt is yours if they were originally a heathen country, but can't it change so ALL released states are same religion? And another thing, could there be an option that you can update their technologies to your level? Its really annoying releasing states like Adal, for example, only to have them 20 technologies behind, unable to even build forts. I like how they take your government type, but can't we also make it possible that they should convert to your form of government when you switch, even if they couldn't adopt that government form at their current tech? It would really make releasing states more worthwhile. Also I guess, on a technical note, you should be able to get causus belli from states bordering your vassals. Its annoying how Holy War wont show up when you have buffer states on your sides
Heretic nations will be released as your religion if you convert their capital (I believe that was the only requirement). The tech levels and governments could easily be handled by events, but that feels a little clunky and possibly broken if a player were to abuse it right. There's also the question of whether the mighty nation of Adal would really progress a century in tech simply because their (probably Spanish) overlords are in control.