Vinyl will also need to be dried completely when put away.. or take a chance of it molding. I my self use a quilt at the lake all the time to lay on the beach, BUT ours is double sided..and only a very thin batting inside. It dries very fast and the sand just shakes off. Vinyl will also make it feel hotter...as it holds the heat in.

When you give him the quilt why don't you also give him the vinyl that fits under it. You can even put velcro stips on each corner and a few places along the border to hold them together while on the ground. When it needs washing you can simply remove the vinyl and wash the separtely. I agree that if you quilt the vinyl it'll have needle holes in it and the new owner may not think about the moisture that could have gotten in it and fold it up and the next time he used it there could be mold. Plus if you wash it the way you talked about making it you risk the chance of the vinyl tearing at the seams plus it would be hard to know if it was dry all the way thru. Just a thought from me.

I think I would just spread a vinyl/flannel backed tablecloth down to put it on.

I think the quilt is a wonderful idea. But make it with a denim back. I don't see how a man who "doesn't think much" about how to take care of the quilt will be able to follow difficult directions for washing it. I also think the Velcro thing is a wonderful idea. That way he would have less to worry about, and his quilt would stay dry longer. Put a few Velcro spots in the middle also.

I actually own this type of thing that was given to me by a company as a free gift. It isn't a full-sized quilt, but pretty big and rolls up into a tight roll then has a strap attached for carrying like a bedroll. I keep it in my car with the tire changing stuff but will dig it out and see what the quilting on it is like. The backing is more like naugahyde than vinyl and I don't think there's any batting inside.

Here's a thought - how about making the quilt top & quilting it & then add the vinyl backing, sewing just around the outside edge. That way you wouldn't end up with tiny holes thru the vinyl that would let moisture in & may eventually ruin your quilt. I like your thought best though about giving him a larger piece of vinyl to put on the ground BEFORE he lays the quilt down.

You can get those flannel backed table cloths that are in the picnic isle at joanns and use that as your backing. it really works great. and you can just lay it on the ground and still wash it.
vinyl might be to bulky,

Don't know if you could use this idea, but the ladies made a rain coat out of a shower curtain.
sewitalltv.com/episodes/series_200/episode_210.html

Rethink this a moment any type of plastic will be difficult to clean, why not try a nylon raincoat fabric of some type. Actually my mom left me a lot of that type of fabric however limited color choices. Easy to clean resists water.

Hancock Fabrics has some vinyl coated fabric for raincoats in colorful prints. I think it's 60" wide. That would work for the back of the quilt and it's obviously meant to be sewn. I have used a WalMart soft vinyl shower curtain to line make up bags I make. It is harder to sew with. I found that it helped to use a foot made for vinyl or leather and a stretch needle and stretch stitch selection on the machine. Good luck! If you do make one with a vinyl back, please show it to us when completed.