When using a mask with glasses, it is best to fit the mask first to get the correct fit with no seal leakage, then place the glasses in front of the nose clip on the mask. It is really dependent on the seal of the mask. Having adjustable nose pads facilitates a better fit than plastic moulded rims. A good seal around the nose and cheekbone will mean no leakage in that region which equals no misting. This is how/why glasses get misted up on the inside of the lens. In our experience most glasses wearers manage to figure a way of fitting glasses with a mask without them getting misted up.
For more FAQ, see Respro® Mask FAQ