If you have any ships, sparrows, ghosts or shaders in your Vault you can delete them. They can all be re-acquired from Collections. If you have any year 1 armor or weapons you can delete those as they would have static rolls and thus can be re-acquired from Collections. If you have any sunset armor, delete it. Even if you like the look of it, delete it because you can make it into an ornament on current gear. If you have any sunset weapons and you typically don't play Crucible or you avoid sunset weapons in Crucible to begin with then delete the weapon. If you have current armor that have low stats delete it. Only keep armor that have decent stats. If you've kept it because you like the look of it, you can still delete it because you can make it into an ornament for your well rolled armor. If you have exotic armor pieces in your Vault that don't have amazing stat rolls or very low stat rolls, just go ahead and delete them. You can always re-acquire the piece from Collections, Xur, or Lost Sectors. If there are exotic weapons sitting in your Vault that you haven't used for a year or more, just delete it. You can always pull a replacement from Collections. Use destinyitemmanager.com and filter for is:dupe to show you all the duplicate items you have in inventory. You can pick an item and there will be a little sub-menu called Compare. Click it and it will show you in a row the item to compare each roll. Compare the rolls and delete the ones that don't have rolls or stats you like. If there are weapon archetypes you don't like using and have rarely if ever use you might as well delete those or delete all but the best rolled version. For instance, if you don't like snipers there is no reason to have 20 Frozen Orbits in your Vault. At this point if you still have too much in your Vault then I would start looking at items in your Vault you have had for more than a year that you've never used. If you've had a weapon or armor piece for over a year and have never used it you're not going to miss it once you delete it.