Hmmm....
When you mix a track, and advance the level of the vocal, that should be reflected similarly in the master, without change....although there are ways to do that, on purpose....or a mastering EQ cut in the mids can take a lot out of the vocal pump...vox being mainly mids. Just mix the vox hotter in you pre-mastered stuff. If the guy who masters your stuff is doing the job so that he's reducing the level of the vocal consistently, to your ears, just jack it up a little, so it'll be where you want it at the end of things. Or master your own.
I'm inclined to believe you might be mixing with a heavy mid-frequency pile up; and the mastering guy, flatening the bulge, is reducing the level of the vox in relation to the bass and upper range stuff. It's something I discovered mastering my own stuff. That's why it pays to get the mix sounding like a lower-level master....if it ain't right-on, frequency-balance-wise, the mastering fixes will screw with the mix.
Hope that helps.
