Robert,
I just lost the gas cap for my 81' and I just went to a NAPA and picked up a new locking one. They may need to order it for you because
most of the NAPAs I went to didn't have one in stock. The after market ones have a shorter key than the later OEM caps and so will probably
fit better. I noticed that there are caps available that take the same key as the vanagon in other places, hence the deeper cap body. Having
said that, however, I don't think that I would have a problem on my van with the deeper cap, but I've never tried it for sure...

Peter