Hello and thank you very much for your message and information 
We already made 5 Year Visa Exemption for me and our kids because they also don t have Vietnamese citizenship, only Serbian like me.
Information which we have is that our kids can not get Vietnamese citizenship because they already have Serbian citizenship.
My wife is originally from Vietnam ( Ho Chi MInh City) and she have Vietnamese passport and from last year Serbian as well.
Do you know what is a condition to get spousal TRC?
I guess that I don t have a right to work with 5 Year Visa Exemption Certificate?
Thank you 
-@Aleksandar BGD
With your wife having a Vietnam passport, I'm pretty sure that she only needs to verify her permanent registered address in Vietnam (always a good idea to double check such things with immigration in HCMC or Hanoi).
That might be her family book (where she grew up with her family) OR someplace where she previously registered herself permanently (some people who move to the big cities from the countryside go through this formality).
Luckily you'll have 6 months to sort that out and perhaps update her permanent address, so it shouldn't be any sort of stumbling block.
Then you (she) will be able to apply for your TRC with a police report showing that address to be her AND YOUR permanent address in Vietnam where she is sponsoring you.
Someone else can answer about the children's citizenship, but as I've observed in other threads, the government definitely tolerates citizens of Vietnam establishing dual citizenship in other countries.
But time and again we see Việt Kiều without passports naturalized as citizens in another country OR ethnic Vietnamese children of Việt Kiều, born as citizens in foreign countries, encountering roadblocks to gaining citizenship in Vietnam.
Based on my limited knowledge of the law, it would seem that a key factor for your children will be establishing themselves as permanent residents in Vietnam, as the laws that do allow for the granting of citizenship here seem to indicate that permanent residency is required.
So my guess is that your wife will sponsor them on TRCs and then apply for PRCs, but the process might be much simpler than that, once the children are physically here in Vietnam.
One requirement APPEARS to be the renunciation of other citizenship (in their case, Serbian), but of course, immigration and a trustworthy lawyer will be able to help you work through the details.
I'm almost certain that you are correct; people here on a 5 year VEC 6-month stay do not technically have any residence status, but are essentially the same level of visitor as a tourist, just for a longer period of time.
So legal employment is probably only possible if you qualify as an expert OR your wife sponsors you on a TRC.
Anyone with contradictory facts or differing opinions is quite welcome to share them.