What ingredient gives vlaamse mayonnaise that special kick?
Frietsaus is just a special mayonnaise

Commercially it is lower in fat than plain mayonnaise

It has oil, starch (replaces extra oil), whole egg yolk, whole mustard seed, sugar, distiled vinegar, and lemon juice

So the kick is just the extra acid (vinegar and lemon juice), the bite from the whole seed mustard, mixed into a lite sauce, not a heavy greasy mayo

BTW: VleminckX Sausmeesters is just a touristy thing, there are plenty of shops in Holland that serve great fries

Personally I think that potatoes from Europe are a bit plain compared to many other continents/countries. the best I have had are from Fiji, the warm humid conditions make excellent deep frying potatoes, and I suspect they lace the cooking oil with a little coconut oil too :-)