I dont think it is anything you are doing or not doing - it is just the nature of the beast!! For the 'simulated stained glass' paints to be thick enough not to drip off a curved surface they are made to a consistency that will inevitably leave brush marks. The only way I know of to not get marks is to paint on a flat piece of glass and use enough paint to allow it to spread out flat - so on a vase you are not likely to get it flat and smooth. I think most people using this kind of paint just accept the brush marks and try to make a feature of them instead of worrying about it. You could try contacting the manufacturer to see if they make a range that is thinner that will flow smoothly but think it would only work on flat glass.