Possible to move visa to new passport?

Hello,

I arranged with you a multiple entry visa some days ago. I am in Vietnam now so everything was ok. But now I have a problem that maybe you can solve.

I want to go to Cambodia so I have to make a new passport in my embassy (I do not have the 6 months validity that Cambodia requires). The problem is that if I do that I will lose my Vietnam visa and I will need it to get it Vietnam again. My embassy told me that there is something I can do with Vietnam immigration to move the visa from my old passport to the new one. Is that possible? Can you do it? How long would it take? How much would it be? What should I do?

If moving the visa is not possible, you think it is ok if I get a new single entry visa with you in the same way I did before?

Or you think I can reuse the letter you sent me the first time to get in Vietnam again?