According to Jamil and George Fix, the slurry in washed yeast is at least 75% yeast, probably more, assuming you washed it properly. And a White Labs vial isn't full of yeast either. There's probably 20-30ml of yeast slurry in a White Labs vial. They claim 100B cells total, so that's 3.3-5B cells per ml. Washed yeast should have 2.5-3.75B cells per ml (probably more). So if you have 27-40ml of washed yeast slurry in a vial, you have about the equivalent of a White Labs vial or a Wyeast pack.