I have often found vending at shows to be a fun, rich experience different from just tailgating before the show. I have met some interesting people, and heard some great comments and stories. I have generally had a good time. One thing that seems to stand out to me is that vendors basically look out for each other. It could be correct change, duct tape, or even "here hold this for a minute", but the things exchanged or shared seemed to indicate there was a larger theme in play.
Online, however, things are a little different. You'll often find one vendor talking trash about someone else's product or other negative comments meant to downplay someone's wares. Anonymity is quite the elixir.
I would like to suggest a better approach. Keep walking. Unless someone is selling the exact same shirt or sticker as you, then competitiveness is not really a factor. There are more than enough cool ideas to go around. Some will do well, others will not. Why spend time and energy to hate, when it's just as easy to ignore them? If you don't like the kind of beer a vendor is selling, do you stand in front of their Easy Up and just yell obscenities at them? Much like shakedown, if you don't like it...keep walking. And if you are a fellow vendor, at least try to appreciate the logistics that got the product from a person's brain to the lot...or the internet.