From what I can see, it looks like you're using an AreaTrigger that runs one of 5 or so randomly selected ShowSpeechBubble scripts.
Add on to that an AreaTrigger that runs on leaving the area, and runs a HideSpeechBubble script, which is directed at all of the possible ShowSpeechBubble nodes, and set to hide last created script. (To do this, you have to right click on the ShowSpeechBubble node, not left click, and select "Last Spawned")
This will remove the bubble when the player leaves (Which makes sense, because then the vendor won't be talking to no-one), which prevents multiple from spawning on top of eachother.
UNTIL you have multiple people enter at the same time, which will run the script multiple times without having the chance to remove any, so just throw some logic in there to only show the speech bubble when the first person enters. That way the vendor won't talk to subsequent customers.