There are many services that are available when it comes to calling overseas from US. There are many calling card companies that offer special rates and one can make good use of these rates particularly if you call overseas on a regular basis.
You can also check with your landline service provider to see if they offer special rates for Vietnam. Generally you can get very good rates even from these companies as they are trying to have a favourable view of them in the eyes of the public in ever growing competition.
Online Calling Services
You can also make use of these. These can either be ones where you pay online and they give you a pin code which can be used to call overseas or you can actually call using the software for which they charge a certain price per minute. These are often quite cheap and reliable.
You can call Vietnam for as long as you like and still not have to pay a ton just to talk to the loved ones who are away from you.
Know the Time
You should know the time difference between US and Vietnam. You do not want to wake up relatives in the middle of the night just to ask how they are doing. You can set panic by just calling at the wrong hour.
Know the Number
The country code for Vietnam is 84. You will need to dial 0084 then the city code and then the phone number itself. The process should only take a few seconds and you will be connected to the loved ones in no time what so ever.