For a price or reputation, you could ask the trader to teleport to a safe backroom, when your timelimit is over, you get teleported back.
For example, in NCR, you go to the trader and when you have 3000 caps in your inventory, you can choose the dialogue option: "i look forward to a bigger deal, may we speak in private?", then you and trader gets in the backroom, where you pay 500 caps and have your time to make a deal without being disturbed by thieves.
On the other side, those who wait to the vendor coming back are the targets for thieves, so it's only fair for everyone.