I think it would be more effective to allow us to pay in order to place actual rewards in our mission. Like it would cost a certain number of c-points in order for us to place a specific item at the end of our missions, or alternatively a level appropriate random item.
Unfortunately, before they do that they need a way to judge how long it took to play a mission, so we don't end up with a bunch of console clickers spitting out loot.
I guess from that standpoint the vendor may make sense, since it's not as easily abused. But really, it wouldn't take more than 5 minutes for "Joe Blows Market" to appear on the Foundry, which would effectively be a "console clicker' that consists of nothing but that vendor. So, realistically, it wouldn't end up bringing anyone to our missions.
I think it would be cool if we could add vendors regardless, though, even if only standard types with nothing special on them.