VJ received sequels for a few probable reasons. Firstly, as I said before, Capcom really believed in the product and it's obvious they wanted to build a franchise with it. Secondly, if you look at the release schedule of the first and second games, it seems highly likely that Capcom immediately began work on the sequel after the original game was finished, They did not take a wait and see approach with it(this harkens back to what I said about them believing in the product and wanting it to be a new franchise). Thirdly, the first game probably turned a profit for them. That does not mean the game was very popular, it just means it turned a profit. Lastly, as I said before, they could re-use many of the assets from the first game for the sequel, thus cutting dev costs.
The VJ DS game and the VJ fighting game were obviously not big projects. Like the two main VJ games, they quickly found their way to the bargain bins. Unfortunate, but true.
If you want, you can look up Capcom's 2004 Annual Report; many times it mentions VJ as "struggling" or performing below expectations and that the sequel is already under way during that time.
Again, I follow the sales of video games and have done so for years. I was actively checking the sales data for games when these games hit the market. I was active in forums when these games (ED and VJ) were under-performing and seeing all of the chiding and excuses made for their lack of success.