Duck adhesive - it works well with vellum!
 

I recently tried the Duck Easy-Stick Double Stick Adhesive with a vellum-type paper. I can't see the tape through the vellum! It showed up a bit pink initially, but the color disappeared the next day.

It also came in a very reasonably priced 4-pack at JoAnne's.

Just wanted to let you know it's worth a try.