just a little name dropping here! but when I took Robin Johnson's vellum class last year, she suggested lots of things.
1) And my favorite idea, is to hide the adhesive under other elements on the page, so put it under photo's etc. It does mean the edges of your vellum may well be loose, but since the page ends up in a page protector, she felt this was ok.
2) eyelets, brads, sew it, any kind of none adhesive.
3) Xyron - this doesn't always work for me though, I tend to get a mottled look. I've been told it's better with the permanent adhesive, and on a lighter background.
4) Spray Adhesive - this is nasty stuff, but Robin said she did use it quite often.
5) you can get clear photo tabs and vellum adhesive, but imho, they are still visible because they change the way the light hits the vellum.