By being at least a little bit discriminatory about the guns you keep in it.
With Transmog and the Collections tab, there is no real need to keep anything in your vault unless it has good stats or good/interesting rolls. And I doubt you have 500 relevant items with such combinations.
Just go through your vault and look at your items. For each one ask yourself, 'am I really ever going to use this again?'
If the answer is not really, then scrap it.
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I do this every season, and delete things that I actually used and might use again because there is no place.. all good.