Check the passenger floor for wetness. Lift the mat and feel if it is damp, if not, rub your hand on the floor and see if you can smell coolant on your hand. If it is wet, heater core for sure. I would look under the hood and under the engine on the ground for coolant leaks also. Check the overflow reservoir and see if there is coolant in it and note the color.

How many miles on the VX in question? What year? Any service records?

Bart