Unfortunately, there is not a way to use previous values or variables in the Vision Assistant threshold step.
The way to do what you want to do is to implement what you want to do in LabVIEW and call your VI from the Run LabVIEW step.
From VBAI, create a global variable or User interface control to define the threshold value. You can use that variable to set the threshold in the count pixel step, and also pass it as a parameter to your VI called by the run LabVIEW step. The Run LabVIEW step also allows you to pass to your VI the ROI that was used in the Count Pixels step.
Implement the threshold/overlay in your VI. That should work.
If you need an example, let me know!
Christophe
