"...the inside of the optical tube is not coated as dark as it could be."
Amen to that!
If you put a bright star at one edge of the FOV, it will illuminate the interior tube wall on the opposite side, and actually put an ugly arc of scattered light in the field of the eyepiece.
I think flocking the interior walls of the OTA (or at least repainting them ultra-flat black) would help. Other users, though, say the culprit is a shiny lip/missing paint on the edge of the baffle tube and have fixed it by flocking the baffle rather than the whole interior.
For my part, I avoid putting bright objects far off axis.
On the vignetting question, what 2" eyepiece were you using (i.e., make, model, AFOV and focal length)? It may be that it didn't have sufficient AFOV to suffer much vignetting with that level of extra focal length in back, and that would be handy to know.