If there is residual ash, then yes, vacuuming will help by removing some of it. But a resin used for casting needs to burn out completely, leaving no residues. If there is some, then you're either not getting the mold hot enough or using an unsuitable resin. Be careful about doing this, by the way. If you suck up hot particles from a super-heated mold, there's a good chance it can start a fire in your vacuum cleaner, which the forced air will quickly accelerate into a raging inferno.

If the thin parts of your casting are coming out well but the thick parts are not, it might be due to shrinkage and not the fault of the resin. Make sure the thick parts of your model are connected by a thick sprue to a substantial pour-cup or riser, so the solidifying metal has still-liquid metal to draw from.

Andrew Werby
Juxtamorph.com