I can't get my husband to talk about planning a vacation without him getting defensive. What should I do?

I would say be open with him about it. Understanding why he's getting defensive will lead you to progress. And once he's noticed that it's a problem, he will be more cautious and aware about it in the future. In my experience, my boyfriend used to always get defensive in some cases, without first listening to me. I started bringing it up later and we agreed that whenever he's not listening and is defends himself, I call him out then and there. Since then, he's stopped (or is at least working on stopping) automatically getting defensive without reason to be, and he's been more of an active listener when bringing up certain topics.