S
Google
There is no public website or remotely easy to understand public information, including regarding opening hours. When I visited, the small gallery on the ground floor was open, even though the sign said “closed.”
What you would really want to see as a visitor here - the 4th and 5th floors - has been completely closed to everyone who is a not a family member of one of the deceased soldiers commemorated by the monument.
(Imagine if a Western war memorial had such a policy...)
On the higher floors, various trinkets appear to be sold, most of which are not from Vietnam.
Essentially, this means that there is little or no point visiting this tower as a tourist. You will not be permitted to go anywhere you would really like to go.
You are better off photographing this tower from somewhere else in the city.