Hi.
Change the point of view of the voltages. Instead VDS, analyze VGD. So, you have VGS, VGD (so VDS = VD - VS = VGS - VGD).
Think about this: A MOS transistor is, in principle, purely simmetryc. (look into the schematic, you can switch S with D).
If VGS > vt, and VGD > vt , (then VDS =...