Have you ever meet them on the street of Vietnam or in a forum?
In someway I can understand why some of foreigners are suspicious when doing something with local people. I think good and bad people are everywhere, we should be careful with strangers in a strange country. However, they're even ungrateful when they've got sincere help from us, which means we've known each other. They behave as if it's our duty to do their favor.
Sometimes they pretend that they are friendly and appreciate the friendship, but I can see they think bad about Vietnamese people, e.g "Vietnamese are silly" (or "stupid"). And I consider it as hypocritical.
I saw a guy in this website often says to Vietnamese like "Piss off! Go back when you understand English clearer!" in a discussion.
Most of them are expats, while foreigners who have just came to Vietnam are much more different.