This is a new bug I've only seen since the latest (Nov. 21/22) updates.
Recharging a vacillator often causes it to reset to an unusable state. It seems to be recharged but the brain is still small, it's unable to be recharged again, and the description indicates that a neural vacillation is currently in progress. Dupes cannot be assigned. Closing and reopening the save game fixes it & the vacillator can be assigned again.
This does not appear to happen every time. I'm not sure why it sometimes works, as the same vacillator will fail or succeed at recharging seemingly at random, although they seem to be failing about 9 times out of 10.
1. Use a neural vacillator. 2. Recharge the vacillator. 3. Neural vacillator will be in an unusable state. 4. Save and close the game & restart the saved game to fix the issue (workaround).