I personally never thought about putting a vinyl backing on a quilt. I would be afraid it would wear out to quickly with as much as I use a quilt on the ground. Little feet or scooting dupas may weaken the stitching or even tear the vinyl.
I have a quilt that is now close to 30 yrs old, a little worn now, but the only quilt that goes to the beach or picnics or for even babies outdoors. If the ground is moist I will either double it up or put a vinyl checkered table clothe under it. I suppose DH and I are one of the few that still use a traditional picnic basket to go to the beach or for a picnic in the park. We take it to go fishing on the shore or for a picnic or to just go to the beach to read a book or listen to good music and enjoy the sun. I think this quilt would have been long gone with vinyl.