The vest has now been washed, and I can now report that it did not wash well!
First of all, I left the vest overnight and it faded on it's own, with no washing, but it still looked pretty good!
Then it got washed, and it's faded even more!! Now I did get confused when I chose the vest, because I thought it needed to be cotton (could have been quilter's brain!) but when I read the instructions (after I'd used it, of course!) it suggested that it works best on polyester! So it probably is my own fault!
I am hoping that it's finished fading now! It does still look ok, just nowhere near as good as it looked initially!