Same thing happened to me back in december. I left the dealership when I purchased the VX and the mechanic did not hook the tranny fluid lines back up after replacing an O2 sensor so all of the fluid ran out and 20 miles down the road I had the same thing you described. "RPMs would rise and not feel any pull" almost like I was in neutral. When I took it into a shop they said it had burnt 3rd and 4th gear down to absolutely nothing so I had to have the whole thing replaced.