So, I've never used either one of these before. I'm going on vacation so I'll put my account on hold. The books that I have on auto request will automatically be accepted during this time, as long as I have enough credits in my account, is this correct?

Books that I don't have on auto request, will follow the normal pattern, if I don't accept them in 5 days they will be gone, is this right?

And, how would I know that one of my books came up on auto request and was then accepted by me? Just by looking in my "books on the way" list? That sounds silly when I write it, but I just wanted to be sure!

You are right about everything.  To answer your question - if you have books on autorequest, you get an email from PBS that says something like "you have requested book x" just like you would get when you manually request a book.  So, that's how you'll know.  You can also access your PBS account even when it is on hold, so that's another way to know.

Actually, not right about everything.  Most of it was right, but for wishlist books that are not on autorequest will not be offered to you; they will just automatically be offered to the next person in line.  You do not lose your place in line, though.  When you come back from vacation you will be placed back in line in the same place you were before (but there may be a fewer people behind you).

http://www.paperbackswap.com/help/help_item.php?id=541

You do not get 5 days to say yes to a wishlist book. You get 2 days. The sender has 5 days to decide if they will send it.