"My dog is very fast."
Translation:Con chó của tôi rất nhanh.
As a matter of fact, the "official" language of Vietnamese is the northern dialect. Whether this decision that has been made long ago is justified or not is a political subject that doesn't belong here. There is no problem in adding south Vietnamese words here, but they should be marked as such so that people who want to learn the official language are not getting confused.
I'll compact my response into a series of bullet points:
- The standard WRITTEN language is based on a pseudo-Northern variety.
- There's nothing wrong with pointing out that the modern WRITTEN language was chosen due to political reasons; this gives context to the language learning process.
- Even so, the spoken variety depends; if you intend to live in the North or be around lots of Northerners then it's completely sensible to stick to just the Northern language but if you intend to speak Vietnamese with those in the South or overseas (where most Vietnamese speakers are Southerners) it's better to focus on the Southern language.
- Regardless of which dialectal variety you choose, it's recommended that you gloss over the differences in vocabulary.
- Plenty of Southern words have become a naturalised part of Vietnamese.
- The example you gave of "mau" is erroneous as that word has been around since prior to Nam tiến (southward expansion).
- More foreigners live and interact around Southern speakers.
TLDR; It's better to learn both at the same time so you can be up-to-date and ready for all occasions. Learning a new word? Gloss over any regional differences. The great thing is that most of the words are shared between the regions.