I think it might be a better question to ask "Was there a Western Bloc equivalent to the Viet Cong, operating to undermine the Communists within North Vietnamese territory?" Because as far as I know there really weren't any. The closest I could think of would be FULRO which was essentially a guerilla group operating against both North AND South Vietnam fighting for autonomy of the Degar tribes in the highlands of Vietnam. But this was not really aligned with the "Western Bloc" (as they were fighting both sides, effectively). Perhaps someone else with a specialty in this subject could correct me on this or go into more detail, but as far as I know: there wasn't an equivalent or comparable group.

However I will go into a bit of detail about the Viet cong and why there likely wasn't a US aligned equivalent. Firstly: the Viet Cong were an evolution of the guerilla fighting Viet Minh who fought for independence of Vietnam, first against the Japanese in WW2, and then the French, before securing an independent North Vietnam under the rule of the Viet Minh. However some of the armed guerillas stayed behind in the South in anticipation of a reunification vote. When Diem and the U.S. blocked said vote (fearing the government of the North would win the election) the North's government reactivated many of the Viet Minh left behind and re-enforced them with the Viet Minh who had previously relocated to the North. This was the start of the Viet Cong, which was subsequently bolstered with local recruitment and even the PAVN (Official army of North Vietnam) who took on an increasingly dominant role after the Tet Offensives wiped out much of the Viet Congs forces.

Simply put: the Viet Cong were derived from the Viet Minh and the Viet Minh were really the only organized and effective resistance movement against imperial powers in Vietnam. When the North and South split, the North was decisively more popular than the South (which was largely viewed as a French, and later US, client state). And with the North's government being the source of guerilla and resistance movements for basically the whole country prior to this split, naturally they would enjoy a near monopoly on guerilla movements. While there certainly were Southern/French/US sympathizers in Northern territory they lacked any kind of organization or structure. It would be difficult to secure widespread popular support for some kind of "anti-north" resistance in favor of the South's government for a handful of reasons:

- The government of the North were the foremost actors in securing independence from France.

- The government in the South were often viewed as a continuation of French/Western imperialism (and were certainly framed this way by the govt. of the North)

- The U.S. would use increasingly hostile tactics in the north (the North was heavily bombed by the US)

So, again to my knowledge, no pro-South anti-North resistance groups ever really formed.