Vietnam hasn’t fought a war in a long time.
Back in 1979 they had plenty of recent combat experience.
Now they won’t have that important advantage.
The Chinese have massively upgraded their tech. While Vietnam, as far as I can tell from published sources, has spent very little on armaments since the 1970s.
In 1979 both were at the same technology state, or perhaps the Vietnamese had a slight edge. These days the Chinese have a huge advantage, particularly in the air.
The Vietnamese may also suffer from a lack of ammunition and in particular of resupply. The large stocks of Vietnam war ex-Soviet ammo are long obsolete. They have no source of resupply (well, possibly India) that would be willing to risk facing Chinese air superiority to get it to them.
The Vietnamese have the advantage of position and terrain as before, possibly negating the massive Chinese superiority in armored units.
In 1979 the Chinese suffered from poor logistics due to the poor roads and rail through the undeveloped region north of Vietnam. I believe this situation has since been remedied.
Well we could send the Viets arms and logistical support. You know kinda like they and Russia did when we fought there.