The south Vietnamese were not communists during TET. When they US abandoned South Viet Nam the communists then took over. The communists in North Viet Nam were supplying the war materials and troops for the war via the Ho Chi Minh trail. The several groups fighting the US troops in the south were all under the direction of North Viet Nam and Ho Chi Minh until he died then things changed a bit. The partizan news media rewrote the news they received form military so suit their political agenda just like they do now. John Hanoi Kerry made stuff up such as Veterans were baby killers and war criminals. Little of the true happenings in Viet Nam made it to the people unspun. When I got home and read the news papers the country that they were reporting was not where I had just left.
South Vietnam had plenty of communists; they were armed by Red China and North Vietnam, had their own units (and flag), and attacked within South Vietnam. The North Vietnam Army itself wore their own uniforms/helmets, and generally attacked from outside South Vietnam (crossing borders to do so).
Our response to Tet left the former in tatters (as the popular uprising they hoped would generate mass support for them failed to materialize), leaving the only hope for victory/unification in the hands of “foreign” (North Vietnamese) troops. That is why in 1972 and 1975 the enemy wasn’t wearing black pajamas, but NVA uniforms - and they rode in tanks.