There is no restriction for payment of cash exceeding Rs. 20000/- for capital expenditure, it is only for revenue expenditure. So you can make payment of Rs. 50000/- in cash to purchase old vehicle provided you are not dealing in sale and purchase of vehicles otherwise it would be revenue expenditure and hence will be disallowed.
As rightly said by CA.Deepak for purchase of Fixed Assets cash payment can be made in excess of Rs.20,000 at a time and no disallowance will be applicable u/s 40A (3) read will rule 6DD.