Should I Skim Coat My Underlayment Prior To Loose Lay Sheet Vinyl Install?

[Jerad]

I am remodeling my bathroom and have run into a few issues that people here have been kind enough to advise upon. I have replaced part of my subfloor and am going to install 1/4" 5-ply birch on top of subfloor as underlayment. I plan to staple this to the subfloor and then fill in the cracks/low spots. However, I got a small amount of flak for using birch under the Sheet Vinyl in a bathroom. Now the FLoor patch I purchased says it protects against mold/moisture because it has concrete in it. It also says it can be used as a skim coat on underlayment using either latex additive or water. My questions is, should I do this? If so, how does one skim coat a floor? Thanks. Floor is 5'x7.5'