It is probably possible to make calls from the US to Vietnam, but it depends on your provider. (e.g. Viettel - yes, Vietnamobile - not possible for prepay customers).
Before you go, you'll need to turn on roaming. Your provider will have instructions for doing this - searching for Viettel Roaming or Vietnamobile Roaming got me what I was looking for, so I guess doing the same for Mobifone or Vinaphone will do the same. You could try searching for CQVT [your mobile provider] as well, which seems to mean Roaming in Vietnamese. They'll probably want you to have a fairly large chunk of credit on your phone, certainly in the hundreds of thousands of VND.
Before you turn on roaming though and call away, as was pointed out in the comments, it will be expensive. Viettel charges you somewhere in the region of 33000 VND/ minute to make calls from the US to Vietnam (that's about $1.50 right now). On the other hand, that's still cheaper than the $3.99 / minute that T-Mobile USA for example will charge you to phone home. Your cheapest option to make calls home is likely to be to use data on a local SIM, and (again from the comments) the question as to how you can do that has already been asked.
It will also cost you a lot of money to receive calls - it looks like Viettel would charge 11000 VND/minute for example - so bear that in mind as well. On the upside, it should at least be free to receive an SMS.
Finally - to dial an international number, you just dial + followed by the country code, so for Vietnam that's +84 (you also leave out the leading 0 of the number).
