DIM has some 'features' that move your items to your vault. These features have apparently been a bit buggy lately and even people that have never used them are having their items moved to the vault. Check in-game (not in the app) in your vault and the items will be there. If you want to check in the app, then please view your vault and press the refresh button a few times until it actually updates to show you your items. If you'd like to stop DIM from moving your items automatically, make sure you go in and turn off all the settings that do so. There's Farming mode and possibly another in your Loadouts section that moves items not in your loadout. There may be others, so just check everywhere.