putting your entire bookshelf on Vacation Hold and putting each individual book on hold?   If each individual book is on hold, will they still maintain their place in FIFO?

I believe the answer is no, they will not keep their FIFO place if the individual book is on hold.  But under Vacation hold, the books do keep their place.

Regarding books on your bookshelf, yes, you are correct, individual books on hold do NOT retain their place in line. If you place your whole shelf on hold, though, they will retain their place.

Regarding books on your Wish List, you can place individual wish list books on hold and you retain your place in the Wish List line.

It's assumed that if you put individual books on your bookshelf on hold, that you are not ready to have them ordered and so should not be moving up FIFO. Whereas if you put your shelf on vacation-you are just temporarily unable to handle book requests. 

For WL books-you can put them on individual hold because you are short on credits, have a different version enroute to you or something. But you want to stay in line.