What is incumbent upon one who vowed to buy a new car with his first salary if that salary is insufficient for it?

The vow you made is of the type that pertains to a permissible act. The preponderant opinion is that a vow concerning a permissible act is not binding, and no expiation is required from one who does not fulfill it. This is the doctrine of the majority of jurists. Therefore, there is nothing upon you if you did not buy the car by the specified time or did not buy it at all. However, it is preferable, if you do not fulfill it, to offer an expiation for an oath, to avoid differing opinions.