As someone who likes to liberate vassals, it's not clear how best to do that currently. I am loathe to donate resources to a vassal given it will make their master more powerful by proxy, who I will eventually have to fight. Sometimes I just to declare war on the master straight up and try to avoid fighting the vassal if I can. That's not always possible though. Fighting the master while the vassal rebels is ideal, but I haven't noticed it happen very often. So tbh my primary approach is the world congress, proposing universal liberation when it becomes available. Unfortunately, it takes a lot of resources to support them enough that they can remain free. Often I will liberate cities in a war only to find that as soon as I peace out the former master declares war on the ex-vassal and takes those cities back because they have low health and no units to protect them. I know I can gift techs and money, and I often do try to bring them back to the present (they are usually a good number of techs behind at that point). Gifting enough money to keep them alive can be expensive though.