just go in your vault and get nasty with it and delete 60 things, you'll feel better for it
start with unused exotics - you can always rebuy them (weapons)
doubles of anything - unless it's god roll, you only need one
any weapons that have high kill counts but you can rebuy - this was the hard one for me - but I can rebuy it so I'm good - though I did keep a couple
my vault was always at max now it's at 440 and I feel good and have room for beyond
I dismantle quite a few things every week to keep the vault manageable. The main problem I have is all the exotic armor that I have collected. I never use this gear, but I am hesitant to dismantle it because it might be useful next season.