August 24, 2016
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Yes, "tháng giêng" and "tháng một" are used interchangeably. At least, my family and I do. We're southerners so I can't speak for the other dialects.
October 23, 2017
I do hear the Hanoi-an uses it more than Saigon-ese, but that is just my experience.
November 23, 2017