seed vigour index II will be very precise way of estimating seed vigour index because seedling length may be influenced by environment, shade and planting density. But, if you want a non destructive method of estimation then u can opt the vigour index I.
Dear Ravindra, if the observation is made on 30 and 60 DAS (days after sowing) then it will be considered as plant height and not seedling length. Formula suggests the seedling length.