Hope this isn't too late.  Don't use polyurethane on your sub floor if you plan on putting down vinyl flooring after.  It will create marks on the top of the floor over time.  I learned this the hard way!!

Sanding and sealing the floor would definitely be a good idea but we left our unsealed plywood subfloor exposed for at least 3 years (don't ask) and are just now finishing the floor installation.  As long as the plywood doesn't get too wet and stays structurally sound, it shouldn't be a problem.  You'll have to cover it with a vapor barrier and underlayment (for hardwoods or laminate) before installing the floor and then you won't be able to tell what was underneath it.

Depending on how long before you install the floor, you could try finishing the plywood for a temporary (or not so temporary) solution like this:

http://www.homeenvy.com/db/1/221.html

Hi YabbaDabbaDelicious,

I can't think of any reason why you wouldn't be able to sand and seal the plywood.  It's purpose is to serve as a sub-floor beneath whatever finished flooring you choose.  Sanding and sealing will have no effect on your future flooring.  A light sanding will remove any splinters and sealing with some type of urethane will make keeping it clean a whole lot easier.

Good Luck,

Suzy