Question: This is regarding the scene where Xander is telling Shaver he wants "All of that, in here" referring to a vast array of weapons to go into the GTO. Where/How in the world did Xander gain access to "all of that"?
Answer: Xander and Shaver were back at the compound where they first met. The one where Xander finds out that Shaver is responsible for making the gun and darts that were used to knock him out. In that scene Shaver shows Xander some of the cool stuff that he had made, and Xander uses a few items later in the movie.
Answer: Vin Diesel chose to reject the sequel as he was being offered three different franchises, based on XXX, The Fast and the Furious concept and the character of Riddick (from Pitch Black) - he presumably felt that he couldn't commit to them all. In the end, he chose to go with the Riddick films and reject the others. The sequel is called XXX for the simple reason that the character is still present - they've just recast the role.