I agree that to paint a glass object in watercolor can be complicated - oh, too many reflexes on it, yes!? :) But, for me, it is the most exciting part of the all painting process. The flow of paintbrush strokes creating the form and texture of the glass vase - isn't that magic!? As you can see in the video, I am painting right away with the brush, which allows me to feel free, rather than coloring within the lines. But the feeling of freedom goes together with uncertainty. I am always unsure about the painting result. I push myself not to fear making mistakes. I am sure the sense of fear is the biggest enemy of the creative process. Do not think about imperfections of your painting. Follow your creative flow - trust it! The painting practice is only a pathway that opens our creative skills.
Take a look at the painting demo video just for inspiration! It is not a perfect painting, but I remember that the 'perfect' is always in its imperfection.