I have never used Vulkem. When I first went to the industrial supply in Winnipeg to get Vulkem, the sales person I talked to said they had Vulkem, but their customers had switched to the Simpson product. The color is grey, and I have used it to do all of the necesssary recaulking on the trailer for the last 3 years. Very easy to work with, comes in a standard size caulking tube, and the cost is $9.72 a tube. It cleans up very easy, sets up quite quickly, and if used sparingly and properly little cleanup is required. I use a nail in the end of the tube and wrap the end with plastic and seal with an elastic band. I have had a tube still useable after 2 years, just sitting in the shop. The caulking around a Vista uses very little, probably 1 tube will do all 5, and I will still have some left over.
This product is not only a sealer, but has adhesive properties as well. I work in the building material industry, so have contacts with many industrial suppliers, which really helps when doing this type of project. And if all else fails, "Ask Andy".