This is a common complaint among Vortex users. There is no solution to this problem except to use an accurate cadence sensor mounted on your bike (such as a Wahoo RPM) paired with Zwift. That’s what I do!
In comparing my Garmin cadence numbers to the Vortex I find that sometimes the cadence is off significantly (50rpm instead of 90) and sometimes it’s quite accurate. The method Tacx uses to sense cadence is simply unreliable. (See this page on Tacx’s site for an explanation of how they compute cadence.)
Since the Vortex senses cadence based on changes in your pedal pressure, the smoother your pedal stroke, the less accurate the Vortex’s cadence numbers become.
If you don’t have another cadence sensor, this is better than nothing. But if you’ve got a different one mounted on your bike, I recommend using it.