if it's your vendor and your house, go stand where you want the vendor to be placed and pick reclaim vendor (or something like that) from your context menu on yourself.
if it's your vendor but not your house, use the house sign's context menu and pick reclaim vendors inventory.
if it's not your vendor, but is your house, you have to wait out a timer (90 hours or something) for the owner to try and reclaim their vendor. After this timer expires it turns into a backpack holding all the gold and items the vendor was holding when it went to the crate. That backpack can be removed by the house owner.