I would rather use Superposition theorem to get the Vt, in following way:
Step 1 : Open circuit Current source and short circuit 20V. Vab would then be voltage across vertical 200ohm resistor i.e. = 50V
Step 2 : Now short circuit 100V & 20V source. Now Vab would be voltage drop across (200ohm + (200 parallel 200)). That will be 2A * 300ohms = 600V
Step 3 : Open current source, short 100V source. Vab would be now = -20V
Thus Vt would be 50+600-20 = 630V
Please let me know if above makes sense
Regards,
Vinayak Bhat