My experiences in the TET offensive of Jan/Feb were VC units, primarily 274/275th regiments then later working Cambodian border (Tay Ninh sector) coming across many NVA. The May 68 TET second phase (mini Tet) South of Saigon was pretty well full Battalion strength NVA units. I don’t doubt some NVA/VC may have combined or fought in the same skirmishes, but no, don’t remember my unit running across both uniformed NVA and typical VC casualties together.
I always thought from reading about the war that the VC and NVA really didn’t fight together very well and there was a lot of rivalry between them.