What am I doing wrong in Southeast Asia?

I'm an American who's traveled pretty extensively in my life, and now I'm in Vietnam (been in this region for a few months). Sometime I've found the general attitude of people to be a bit confusing.<br><br>For example I'll be walking down the street and vendors will see me coming and shout "Hello!" at me, but if I respond with a "hello!" or try to initiate a conversation they immediately shut down. I've even had them literally turn their back on me when I approach to look at their wares. I've tried smiling, being polite, even speaking some basic Vietnamese and nothing seems to work. Even if I do manage to find someone who is willing to talk, the conversation usually ends after they realize I'm not interested in buying anything.<br><br>I thought maybe it was just being a tourist (even though I've lived here for a few months), but my friends who are Asian have also mentioned having the same experience. I tried to think back to all my other trips and can't think of anywhere else were the general population was so seemingly cold.<br><br>I don't know if it's a cultural thing, but I'm getting pretty sick of people being rude.<br><br>Edit: I should mention this is mostly older people, particularly street vendors, that I've noticed this with.