Virtual Vietnam phone number or mobile number?

I'll be traveling back and forth between Vietnam and the states quiet a bit. When I usually come to HCMC I just buy a local sim card to use for minutes and data but it seems like the cards expire if I'm away for more than a few months.

I do business here in Vietnam so I'm looking for a way to keep a permanent HCMC number as cheap as possible. So I'm wondering, is there a good service for a virtual phone number that I can purchase and forward the calls to any new sim cards I purchase in the future? or a do you recommend a different solution?

My first thought was Skype as I have one for the UK, but they don't have such service in Asia.
Google throws up a lot of results but their quality is unknown to me as I've never used any of them.

I believe most of the major carriers let you keep an inactive number alive for a small fee.  If you are in VN now, just ask them.  I don't know about forwarding calls to the US.

Skype does work in VN, but the service quality used to vary.
Mobiphone accounts do not work outside VN.

Arranging to have someone in Viet Nam make one phone call/month on your phone and making sure that you keep the bill paid should pretty well guarantee that the number does not change.

Skype and Magicjack do work in Viet Nam. But, you need the best internet connection that you can get. Get Fiber Optic if at all possible.