Typically from the time NASS decisions are made and through the following year. Supposedly, candidates are only to be offered NASS or CVW, not both. Sometimes there are errors and sometimes a CVW might be offered, in addition to NASS, for extremely competitive candidates that are on the fence with a decision to accept an appointment (or who have an LOA). Again, there is PLENTY of time (almost a whole year) from when NASS decisions are released to attend a CVW. It only use to be that seniors were offered CVWs...then it became juniors in the spring (after they opened a NASS application)...apparently, now it is happening before NASS applications.
It sounds like admissions screens some NASS applications (the ones not selected to attend NASS) for CVWs...which is why I say to wait. It is possible to be offered either/or in the spring.