Not sure what the B3 will be. By the way, B3 visas no longer exist...
I have heard that you need to leave the country and obtain a new visa because of the changes to policy on 1/1/15. But I might be mistaken.
The company is Thuan Viet Travel, 48 Nguyền Thiện Thuật, Nha Trang.
their phone number is 0586252024.
[email protected]
I shopped around quite a bit and they seem the most helpful and friendly too.
I'm sure they can advise you.
Oh, and I got my visa on arrival from vietnamvisapro.net
They were also very good and cheap. The letter only cost $23. Then the authorities get their $45 at the airport.
Hope this helps.
If anyone else has come across good prices elsewhere, please post to this forum.
Thanks