I think about set up new site for DNN community in Vietnam. And the new domain site can be www.dotnetnuke.com.vn. However, I don't know if it is legal or not. Can you confirm me about this, please?

The  question to ask your self is why would you want to use the Dotnetnuke domain name?

The answer to your question - is if the trade mark is not registered in Vietnam then you can make application to seek the term Dotnetnuke as a trade mark - you will no doubt have an objection raised against it by the Dotnetnuke Corp. If you were successfull in gaining the trade mark in Vietnam then if you hosted your site in the USA and I assume Canada then the Corp would probably pull your hosting company into a USA court - if you hosted it out side the USA then again the Corp could attempt to bring the matter before a USA court but it would have small teeth in forcing any out come of the court.

If you register the domain dotnetnuke,com.vn then there are in place international remadies if you porport to be Dotnetnuke or to trade on their Domain name good will  your registration could be made void.

Again the question is why do you want to use the dotnetnuke term - if say you are providing dotnetnuke support then you would have no problems using www.dotnetnukesupport.com.vn or such like - this is not trading on the goodwill or porporting to be the Dotnentkuke Corp if you actually provide support to users using dotnetnuke

The Corp has a system in place for those that wish to use the Dotnetnuke term - the truth is that outside the USA it is a ''voluntary system' as the Corp has still not taken global trade marks or patents on the term however with that in mind I would first approach the Corp and talk to them about what you want to do.

Take for example www.dotnetnuke.com.au which appears to not even use DNN for its site software and further its portfolio clients listed also appear not to use the DNN framework?