Is it permissible to fulfill a vow of slaughtering a calf by slaughtering a number of sheep equal to its material value and quantity of meat?

It is not permissible to substitute what you have vowed with something else, except in cases of inability. If you cannot find a calf, it is permissible to substitute it with its equivalent in camels, or with seven sheep from the age and description of sacrificial animals. If you slaughter ten sheep, there is no doubt that this is better than a calf.