What's the cost of living in Vietnam?  

This depends on the person. If you're trying to live the bare minimum in HCMC, there are hostels that go for roughly $3 a day. That's $90 a month for a bed. If you eat 3 times a day you can find street food or meals that locals eat from $1-2. So lets say $6 a day for meals. That's $180.

Total around $270 a month. But then you have to consider entertainment, wifi, etc so lets add an extra $100+. $400 a month should be bare minimum. As a foreigner, I recommend at least $700/month though. It will be cheaper outside of the major cities though.

Yeah, $700 or so a month is not bad at all. I currently pay $1000+ a month at my current place. That accounts for my rent, electricity and such. But that's also not accounting for food, clothing, and everything else.