The VX tranny doesn't have an overdrive gear. You are probably thinking of the lockup torque convertor engaging and disengaging (when it hooks up, your revs will fall, but only about 1/3 as much as a gear change would cause the revs to fall - 500 rpm vs. 1500 rpm).

From what I can tell, some VX torque convertors are very obvious about locking and unlocking, others not at all. It should lock up in 2nd, 3rd, and 4th gears. Are 2nd and 3rd still working correctly on yours? If not, have them check whatever could affect the performance of the torque convertor lockup mechanism. I'd guess that the wrong spec tranny fluid might cause that.

Good luck!