This has been said a billion times before, but if they would just allow general inventory items to stack higher than they do (+20 for ammo packs, +200 for destination mats), most of my vault issues would be solved immediately. I have mats and ammo packs taking up the vast majority of my vault space because of this.
If display memory is the issue (interactive # of items on the screen at one time), then this would make the most sense. Instead of 5 ammo stacks of 20 each having to be displayed on the screen, you'd have 1 with 100. The only thing that changes is the # "20" to the # "100" and you can open 4 slots in your vault (or character inventory). Not only that but I am constantly at 200 marks on each character, and with no weapons left to buy, I have to keep buying mats... this seems to be taking up more space than anything else. I have more of each than I'll use from now til Destiny 7. Most of the time I just stay at the cap and lose out on gaining additional marks because I can't hold anymore mats.
Increase the stack count max for this stuff. This has to be the easiest solution.