The value set in the load script will have priority. The problem is how to accept an override from the command line. The method I have used is to use a second variable (vcFlg2) in the script like this.
LET vcFlg2 = if(len('$(vcFlg)')>0, ('$(vcFlg)', 'N'); // Take variable from cmdline if provided, otherwise set default
SET vcFlg=; // Reset variable to null
.. then use vcFlg2 in the script