Does soaking fries in vinegar before blanching work better than blanching in vinegar water?
I have not come across this pectinase idea before.

Acidity only inhibits the enzyme activity. Would it not be better to denature the pectinase with heat (and acidity as insurance)? So, a short dip in a very hot acidic bath is better. I do not have the temperature for pectinase off the top of my head, but I imagine that it would be 70C or higher, very briefly. But bear in mind that acid will also inhibit maillard and caramelisation.