I have played around 40ish hrs in the game and I loving it.
I am playing with 3 friends Co-op. There are many times when we have to interact with the same vendor. Its a bit annoying that we all cannot interact with the same vendor at the same time. Each player takes atleast a few mins browsing and performing upgrades/purchases, which can increase the wait time for other players.
It would be nice if all players could interact with the same vendor at the same time, I see no reason why not to keep it this way. Every other game has vendor systems implemented this way, I understand the imersion factor but waiting 15-20 mins to get to a vendor in a 4 player lobby is a bit tiring.