Then why wouldn't 'vo' be used also as a vocabulary word. I have never heard of "tat' before. It's fine to use words that Northern people use, but people learning Vietnamese should know the different vocabularies that exist in different parts of Vietnam.
The Vietnamese course does teach Southern vocabularies. By the way, "vớ" is already accepted.
I think the speaker pronounce it too quick, it sounds much more like a tắt to me