if it's your vendor and your house, stand where you want the vendor placed, and use the context menu on yourself and pick claim vendor (or something along those lines)
If you're not the house owner (ie. it's a rented vendor on someone elses house) click on the house sign and open up the context menu and pick reclaim vendor inventory.
If it's your house and someone elses vendor you have to wait for a set time that the owner has to reclaim the vendor. (like 1 rl week I think) After that the vendor will turn into a backpack in the moving crate and you will become the owner of everything in the bag.