Trusty, reliable & invisible vellum adhesive
This is great for adhering vellum and it is 99% of the time transparent (maybe once or twice did this show through vellum for me). You'll need a pair of scissors that can get sticky stuff on it (your "secondary scissors" if you will, not your super-prized possession ones). Cut this in pieces & affix to the surface you'll be sticking your vellum onto; do not put on the vellum itself. Peel the backing off the pieces, place your vellum & voila, invisible hold. When I ran out of this recently, I purchased 3L's Vellum Tape just to check out & it was horrible, completely showed through vellum. So I came running back to this tape! This is a go-to product for me.