Welcome to the Forum Nutrastat!

I am having trouble following your description. Not sure what you mean when you use the word "post" because you seem to be using it to mean more than one thing.

I can't tell if the crux of the problem is the data validation will not accept null, or a problem in the VLOOKUP, or a problem in an IF formula.

This is where the validation cell will not accept a null string (i.e. "").
You can modify your validation rule to expliclitly accept the null string. In fact, the cell should be blank before the user selects anything 1...100.
VLookup formula
Please show the formula
the IF formula
Please show the formula

BTW consider using XLOOKUP instead of VLOOKUP.

It sounds like you have a large file but this will go a lot faster if you can attach your file for analysis.