China would have been better off leaving Vietnam divided.
One end capitalist, the other end communist (a buffer state), both sides looking at each other from across opposing sides of the border.
This two country approach works well for China with the two Koreas.
Vietnam is a country in which the communist government has stamped/driven out any internal dissent so now the the Vietnamese Government can fight back on external threats if needed be.
Oh, if the Chinese in the 1960’s could see into the future how unfriendly and uncompliant a united Vietnam could be I don’t think they would have throw so much support in toppling the Republic of South Vietnam.