By all accounts, cost of living in Vietnam is very likely much cheaper than you are used to back home. In the major cities prices will inevitably be a little higher but still very affordable. Go a little further out and it gets even more inexpensive. However, if you have a larger budget, Vietnam also has luxury accomodations and services. As expected, rent in the larger cities will take up a considerable portion of your budget but often includes maid and laundry service and is still cheaper than many other cities.
One of the best deals in Vietnam is the food. There are restaurants and cafes offering delicious dishes for a few dollars, some even offering free refills of Pho.
Transportation in Vietnam is also a bargain: taxis, buses and local flights are pretty affordable. If you are feeling adventurous you could rent a motorbike and try your luck at navigating the streets like a local.