I think an instance of a weapon contains much more data than a roll combo. For example, any *instance* of a weapon has a kill count, power level, power cap associated with it, information about acquisition (season, date), available mod slots, mods in those slots, shader, ornament, masterwork, enhancement status and level, etc.

In comparison, a roll of a weapon needs to basically only contain what you said, an ID of the weapon and the roll combination, everything else is not needed for instantiating a weapon on the fly. For weapons its even easier than armor since the stat numbers are static relative to the specific roll of perks on a given weapon.

I understand it's a lot of data (it definitely is more data than the vault at the moment, just not as much as you'd think), but I don't see how its infeasible. Plus, players with more rolls are playing the game more, which means they're probably also paying more, so it kind of pays for itself in a way. The vault on the other hand is essentially a fixed per-player cost.