I can't imagine going on vacation if one of my pets was very ill. First of all, it's completely unfair to ask someone else to deal with caring for a sick pet. That's your responsibility. Secondly, if I knew one of my pets was likely on their last legs, I'd want to enjoy what time I had left with them. You have every right to be upset with your friends.
That being said, you're doing an amazing job and deserve some kind of pet saint medal.
Jennifer and the kitties, Coda and Caligula