I am a water color artist, and in my experience vellums and calligraphy type
papers do not work well for water colors. I really don't think priming, or
stretching would do any good only because the papers are not thick enough to
take that amount of water. If you are going to
want to use watercolors I would reccomend a watercolor paper, for textured
use a cold pressed, for smoothness use a hot pressed paper. You do not need
to stretch it normally for small projects.
Hope that helps!
Emily