I think an anti-rub vest should be fine on her, though I use the quilted ones rather than the spandex as I tend to get wither rubs using the spandex ones.
But a good cover shouldn't rub her anyway. A friend of mine had her big guy go through a fence and get a fist sized hole in the front of his shoulder that got stitched up. He wore several layers of rugs and never once got the stitches or the rest of the wound rubbed.
But if she isn't one of those horses that feels sorry for herself as soon as it starts to spit, then don't worry about a cover just yet. Just do what Kayty suggested.
My boy has to be rugged, he grows no winter coat to speak of, and loses weight like it's going out of fashion.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms