The red hand stamp on this 1969 cover ominously warns, NHO TET MAU THAN / DE PHONG XUAN KY DAU - "Remember the Year of the Monkey / Take precautions for the spring holiday" (year of the Rooster) .
The Year of the Monkey, of course, was 1968, and Tet was when the Vietnamese Communist forces launched a full-scale offensive against the Republic of Vietnam.
A smaller version of this hand stamp exists with the same text and there is a third version that specifically recommends uniting against the Communist enemy. They are all illustrated in Richard Aspnes' excellent Commemorative Postal Markings catalog.
The rectangluar KBC 4416 cachet is also interesting. I haven't seen one quite like that and I didn't previously have a record of this KBC number.
Update June 8:
I received a response from knowledgeable collector/dealer, Anh-Tuan Tran, who specializes in cachets and markings on Vietnamese covers. He believes both cachets on this cover are fake. There is not a lot of fake KBC material on the market, so I admit I did not consider this possibility.
The back-stamp postmark in Vinh Long looks good - the March 2 1969 date is late for Tet, but it's logical this marking may have been used for a couple of months to keep people alert. The QUAN -BUU postmark is illegible. Like I said, I haven't seen this type of KBC cachet.
Anh-Tuan provided a scan of another "Remember the Year of the Monkey" cachet for comparison - this one an M3 First Day Cover. As you can see, the letters in the hand stamp are much more precise and clear.
Is the KBC 4416 cover a fake, or was the hand stamp regionally made and applied during the contemporary period?
Based on the available information, I think the underlying cover is genuine but I concur the two hand stamps appear to be a fake.