Hi Pat,
I don't know, of course, but my friend Joe is on a vent right now and they are trying to wean him OFF it during the day, afraid he will get too used to it early and not be able to breathe without it before he truly needs it. I wonder if using the bipap early might not be like that: if you rest your lungs, you might be letting them -- more your diaphragm muscles -- get deconditioned rather than actually being restored. Since a muscle works better the more it is used, it only makes sense to use your own muscles to breathe for as long as it is possible before resorting to a machine.
Remember, though, that this is only my opinion, and based on what I know about vents and breathing. Hope my two cents helps you make some decision, but that you base it on a lot of two cents from everyone!
My best to you,
Pam W