Thanks to the awesome Google skills of Book Glutton in response to this post about science fiction author Michael Vyse (you can read about BG's research in the comments on that post), I think we can safely determine what actually happened to Vyse. Book Glutton found this link detailing a couple called Michael and Margaret Vyse, now living in Normandy, where Michael paints and does wood carvings in relief. Could be an entirely different couple, you might think, but Vyse did design the jacket for Overworld, his only novel (he also published a collection of short stories, The Outer Reaches), so it's not unreasonable to suppose he might have turned to art instead of writing. But here, I think, is the clincher: last night I checked my copy of Overworld, and lo and behold, there was this dedication (click on the picture to enlarge):
So there you have it: Michael Vyse, alive and well and happily painting in France.