Is it permissible for the Muslim who benefits from his vow (meat, hide), if he vowed to slaughter a calf for Allah for the healing of his son, and is it permissible to combine the vow with slaughtering the calf for a wedding feast?

It is not permissible to eat from a conditional vow (Nathr al-Mujazat), which is a vow contingent upon the realization of a specific matter. Rather, it must be distributed to the poor and needy. It is not permissible to benefit from any part of it or to make it a wedding feast.