When it comes to Vo2 i would be less concerned by dialling down the workout intensity so long as your heart rate remains in that zone.
Vo2 is about processing oxygen and working the heart, the precise number of watts to do this can vary. Vo2 is probably better done without erg mode too where you can push higher watts to get the heart rate up and then just push as hard as you need to to keep the heart rate high. Failing to keep up with erg mode and bailing out of intervals could be the wrong thing to do as you were probably in the right heart rate zone to push your vo2 capabilities despite pushing lower watts.