Using tap water? D.I. water?
Is the rust right on the surface of the steel, and how fast does it show up?
How old is the coolant, are you adding coolant to water when mixing....is there hip hop music allowed in the shop (that caused my tool box to be rusted shut until it was stopped)...and does Vasco need a supplemental rust inhibitor?
It's not real uncommon for rust to form in between surfaces, like between the base of a vise and the table on a mil...where you can get anaerobic conditions and stinky too.
I've had similar, though much less severe rusting with Master Chemical E206. Good product, have used it in both turning and milling, not without its flaws.
We bought a reverse osmosis system to provide water, and that solved several problems, like increasing mineral build up from evaporation, and eliminated the de-ionized water chemistry issues.