I am about to go on vacation for almost a month, and have this question: What happens if I am offered a book on my wishlist while my account is on hold?

You will be passed over for wish list books while your account is on vacation hold unless you have the wish list book on "auto request."  An auto-request book will be mailed to you even if you're on vacation hold as long as you have a credit to cover it.  If you don't have a credit, you'll be passed over for that book.  When you are passed over, you keep your spot in the wish list line.  You can even move up in the WL line while on hold if people ahead of you are offered books.