Need help with an overbag to combat damp down bag

I know there's a lot on the forums on overbags, but I need help understanding what materials (insulation and shell) are best for an overbag, and whether or not they really help keep the moisture out of the main down sleeping bag. Do they really help push the dew point out of the down? Or are VBL's more effective in keeping the moisture away from the down? Any help is appreciated!