i find 10 minutes rather long for the quality you get. My clay setup is the next:
vray default lights off; gi off; noise treshold 0,002; override max skylight + reflections on, skylight 0,0 reflection 1,0
i make a car with plane underneath it that covers all the area, all grey mat.
1 vraydome about 4 carlenghts above the car with a multieplier of 1,2, subdivisions set to 100.
I get good quality clay renders in 7/8 minutes without artifacts or strange shadows. resolution was 1200 x 900.
You should give it a try
really simple but effective setup. Hope it helps for you!