With the new QoL MK2 upgrade, neural vacillator has no priority category. So even when setting priority 9 on the neural vacillator, your dupe can't use it if he has any category on higher priority than normal.
Here is a screenshot showing the problem.
Steps to Reproduce
Have a higher priority than normal on any category, dupe won't use neural vacillator unless he has really nothing to do.
This issue has been fixed in either a future version or updated in a previous version.