https://en.wikipedia.org/wiki/China_in_the_Vietnam_War
The great friendship between China and the USA started after that border conflict, as China was looking for a counterweight.
https://en.wikipedia.org/wiki/Sino-Soviet_border_conflict
I mean Mao was playing Nixon and Kissinger against Brezhnev.
The funny part is where Kissinger is taking all of the credit.
The Sino-Soviet split had already happened (as you mentioned) and North Vietnam played China and the USSR off against each other "who can be the best friend of an important communist revolutionary government?".
It wasn't until the US engaged both China and the USSR in detente, and the war had progressed to such an extent it was obvious the US was leaving, that Vietnam moved way down the list of priorities for both the USSR and China. Both countries continued to support North Vietnam, but it was heavily tempered by what they might lose with the US.