This stuff is not commonly sold at your average paper/office store and is usually sold as drawing Mylar, not vellum. However for plans that require greater long term storage and dimensional stability and that are intended for scaling off the drawing and for writing on with pencils some drawings are done on Mylar with a matte finish that looks very similar to vellum with similar translucency. In general vellum of the 'drawing' sort is usually 100% cotton. Given that at least two of us have tablets marked either 100% rag or 100% cotton rag, and given the above excerpt, what basis do you have for saying that modern Vellum is NOT 100% cotton? Just interested. Some brands of writing-paper and other sorts of paper use the term 'vellum' merely to suggest quality, when it is actually not vellum. Usually translucent, paper vellum is often used in applications where tracing is required, such as architectural plans.

Known as paper vellum, this material is considerably cheaper than animal vellum and can be found in most art and drafting supply stores. A modern imitation is made out of cotton. Today, because of low demand and complicated manufacturing process, animal vellum is expensive and hard to find.

I believe some varieties contain acrylic.? But I'm not sure. Modern Vellum is not 100% cotton, and although it feels plasticky (I assume you are talking about a translucent variety), I don't think it is. All of this is clearly confusing, but to answer your specific question.

Gilclear translucent paper