Mono in Glasgow is great. I was there yesterday, and its good food and drinks, all vegan.
I've never been to Stereo, though I've heard good things about it. I think its the same owner/manager as Mono, as well as The 78, which is another vegan cafe/bar.
For Edinburgh, I'll second the Baked Potato Shop, its great. David Banns is nice as well. Another place I like is the Engine Shed Cafe, which is good for cheap lunch and cakes, though I think it will be closed over Christmas/New Year. Also, Kalpna and Ann Purna do great veggie Indian food.