you can get a reasonable headset for use with Skype and the like for 60-80,000 VND. I don't know why you'd want to spend more considering the sound quality of VOIP.

These are readily available in any computer shop (Ly Nam De or Le Thanh Nghi).

I would recommend any of the USB(not analog) DSP Plantronics headsets. I am not sure if you can get them in Vietnam but I used Plantronics headsets for the last 8 years for both VoIP and PC gaming. Sennheiser is also good, I have never owned one but tried them out and they seem very good and I've seen someone using Sennheiser earphones here so they might be available here.

I'll second grubby - 100,000 is the limit you'll want to pay for a set of headphones with a VOIP quality mic.

The big red shop at the southern end of Ly Nam De has a good selection and unpushy(?) staff.

Hey Grubby,

I thought, and I have no idea, that if I bought a nice headset it would improve the sound quality. Is this not the case??

the bottleneck is the amount of audio data you can transmit reliably over the internet.
You should invest in headphones for listening to music that you have on your computer; phone conversations are very low bit-rate, therefore it's a waste of money to buy any but the most basic headset that works.