Communism spreads by propaganda and the communist armies often seem to be quite well indoctrinated, which may have lead to a self delusion that the populace of South Vietnam was ready to launch a populist uprising. In essence the communist leadership listened to their own propaganda. You can see the same effect happening in some circles on social media today. Whereby the individuals concerned create their own echo chamber and then get very surprised when the real world tells them they're wrong.
With this focus in propaganda it's no surprise that during the Tet Offensive one of the locations targeted by the VC was the National Broadcasting Station, for transmitting radio signals. Knowing the importance of the location, several years before, the VC bought a house about 200 yards from the radio station and used it to stockpile weapons. This wasn't a specific plan for the Tet Offensive, just done on the idea of controlling the propaganda stream.
In actual preparation for the Tet Offensive the VC trained up a special technician, he was to carry tapes containing a call to rise up from Ho Chi Min, which was to be broadcast from the radio station.