Is it permissible to make a vow to abstain from a specific sin, with an expiation (such as fasting or charity) attached to returning to that sin, given one's ability to commit or abandon this sin?

Undertaking a vow (nadhr) is reprehensible (makrooh). What is mentioned in the question is called a "nadhr of contention" (nadhr al-lajaj). The preponderant opinion regarding it is that the one who makes the vow is obligated to offer an expiation for an oath (kaffarat yamin). One should strive against their soul and distance themselves from sins, and not resort to such a vow.