I read somewhere, from a mysterious letter I found in a box somewhere, that VM&P naptha recently (as of october 2015) started adding something to the naptha that leaves an iridescent film on a glass plate once the naptha has evaporated. I haven't tested this theory, but it was suggested in that letter that the problem can be solved by acidifying water and adding the naptha to it, then pulling the naptha off and repeating the process with alkalized water.
The last naptha I bought, about a month ago, had been manufactured in 2014, so I don't know if anyone else is experiencing this problem as of yet.
I know this was sort of off topic, but it has to do with naptha.
If you can't find VM&P, or another naptha of equal quality, maybe you could buy a few bottles of zippo fluid and find a way to remove the impurities from it? I don't know what kind of process that would require, but it might be worth a try. You might even be able to find a way to clean out coleman camp fuel, and that stuff is super cheap.