I found this thread because I was looking at lights for the ARB bumper. It looks like the lights supplied by ARB are a Chinese copy of the original VW lights. The original suppliers are ULO, ZKW, Sidler, and Hella. I don't know if those seal any better than the ARB ones. I'm guessing they probably do because I saw some old used ones on eBay and they look pretty clean inside. The VW part numbers are 171953055D and 191953055A for amber and 191953055 for white. The lenses are 171953141C for amber and 191953141A for white.
The current Hella part numbers are 2BA 002 939-171 for amber and 2BA 002 939-275 for white (the white one appears to be available from VW only); and 9EL 116 771-031 for the amber lens.
If you want to find a particular original VW version you can look for these used or NOS (all amber):
-Hella is 2BA 002 939-035 with VW part # 171953055. Marked 171953049/B/C. (White version is marked the same.)
-ULO is 3320017. Marked 171953049F.
-ZKW is marked 171953049/J.
-Sidler is marked 171953049E.