The thing having a vassal does for capping success is it combines their power ratings when determining whether you can cap them or not. It makes the master quite a bit more resistant in practice (often refusing to cap all the way to elimination if you ignore their vassal), but strictly speaking is still completely possible if you are powerful enough. You can even capitulate former enemy vassals without firing a shot against them (no war success at all); it happens quite often to me in Earth18 games with Tokugawa and Mansa who are always leeching off somebody the whole time. When their master caps they often follow suit the same turn as they are so small. Not sure if it has to do with Power Ratings or how much you've beaten on their master (in the XMLs, the vassal doesn't care about war success against the Master when determining to break away or not, so I doubt this) but it is possible to do.