Is it permissible to fulfill a vow in installments, and should the vow be for the full salary or after deductions?

You must pay out the first salary in fulfillment of your vow, as fulfilling a vow takes precedence over paying other debts like rent and bills. Sheikh Ibn Uthaymeen mentioned that if the debt precedes the vow, the debt takes precedence, and if the vow precedes the debt, the vow takes precedence, unless the vow is for a specific thing, in which case it takes precedence. You are not allowed to pay the vow in installments; rather, it must be paid in full immediately, as long as you are able.