Vellum used to be made from animal skins, but the papers we use today are imitation vellum, made from plasticized cotton. Vellum is a semi-translucent paper, and that quality makes it difficult to adhere to other surfaces because adhesives have a tendency to show through.
These days, you can buy a vellum adhesive tape to adhere, and other options include using brads & eyelets to attach it to your other papers. I’ve had great success using Elmer’s spray glue. Using this type, you have to spray quickly and evenly, but it dries clear. If you happen to use an adhesive that shows through, try some quick fixes, such as inking the paper where the adhesive shows through, or adding a ribbon or another trim to that space. Experiment and see what works best for you. Don;t shy away from using vellum. It adds such a beautiful quality. This card features a picture I printed on vellum.