COTS (commercial off-the-shelf) Validation FDA Requirements
The validation of the vendor won't be useful (perhaps food for thought if you had it).
The validation of software is a classical IQ OQ PQ.
IQ is verifying installation and configuration (the VB script may be seen as part if the configuration),
OQ is verifying sw functionalities one by one,
PQ is verifying sw with real use scenarios.
OQ sometimes is done on a testing platform, different from the target platform used for PQ.
Testing the workflow in your procedure sounds more like PQ.
Before PQ you need an OQ. To do so write a Software Requirement Specification document (many templates are on the internet) containing the functions you use of the software and probably the custom features of the VB script. So when you're thinking of gathering only the functions used at your company and test them, you're right.
Btw, don't forget in you SRS to write the functions used by technicians who maintain the software (eg: backup/restore, calibration ...).
In terms of documentation, you'll have at minimum a SRS, IQ/OQ/PQ protocols and IQ/OQ/PQ reports. One very important thing is to record in your reports which version of software you tested (consequence: the VB script should probably be version controlled).