I didn't have any problems at all and I used the vendor multiple times. Villain side though, so if it was the hero one it may be specific to him.
Mine was Hero PvP Phase. Literally nobody around as well. Had another person in the group have it happen.
Had no issue with the vendor on both PvP and PvE side. I did notice a bit of lag in the 4 man op in the open world though. I also crashed exiting my lair out in to Gotham.
I didn't have an issue with the vendor crashing, but what I did get was hangs when scrolling any vendor. It's possible they are related to the same thing. Also, I do not like the changes to the font and UI text for the gear. They are bigger and makes it so you HAVE TO scroll to read things. I hope it is just place holder and not the new way. The old [+4] or something next to the stat read better.
No issues for me on this one, no reported issues from league mates - I tested the hero and villain, in both PvE and PvP phases - will try every day to see if it happens.