Well the Vatican is not a second rate prize either ...
But it is not a decision maker in the EU, if that is the intended target of those visits.
Maybe Charles and Camilla's trips to Austria, the Vatican and Romania are indeed of a different nature and not so much related to Brexit, although, in the case of Romania specifically, there must be some connection to the fact that Romania is one of the EU countries whose citizens are most likely to be targeted by any post-Brexit crackdown on immigration to the UK.