Well there are no caps out for the vx2 carts yet, so nothing? I read in a review somewhere that certain parts of the 510 and VX2 were interchangeable, the carts being one of them. So maybe buy 510 carts that have a cap and use that? Don't blame me if it doesn't work though, I really have no idea.
I only carry one cart with me and a bottle of juice to top it off with.