[Audacity-translation] Vietnamese translation update

[Brad <bra...@gm...>]

Good day to you all!
Here is the updated  Vietnamese translation for Audacity.
I hope it can find itself in the next 3.1.0 release

I'm proud to say our team got it over 87%  translated these days and it's
already been making a huge impact for countless users here in Vietnam.

Thanks to all of you volunteers for your work.
Cheers!
~Brad