Following the advice of Dave Renfo in comments, I looked up the 1990 interview with Tụy in the Mathematical Intelligencer. The article is fascinating, and I'd recommend it.
Sadly, it gives a pretty brief account of the founding of the Society, and none of the journal.
Koblitz: And you were able to do mathematics during all
Tụy: Yes, in fact our general morale was so high
that we continued our seminars on a regular basis
during this whole period. The Mathematical Society,
which was founded in 1965 by Lê Van Thiêm (I was
general secretary), organized joint seminars in optimi-
zation, probability, functional analysis, algebra, nu-
merical analysis. People from Hanoi University, the
Pedagogical Institute, and the Polytechnic Institute
participated (the Hanoi MathematicaI Institute was not
formed until 1970). Since the three institutions had
been evacuated in different directions from Hanoi, we
held the seminars in Hanoi. They met twice a month. I
must say, people were very diligent about attending.
Many of us would take advantage of the opportunity
to visit our families. Since my wife and children were
on the opposite side of Hanoi from the university, it
was much more convenient for me to visit them after
the seminars held in Hanoi.
He has a longer description of what it was like trying to run the University at the time, after it had been evacuated to a spot in the forest some 170km outside of Hanoi, having to teach classes near trenches they could jump into if the Americans started a bombing run, etc., and some amusing anecdotes about Grothendieck visiting the society and having to be evacuated due to bombing.