What should a woman do who vowed to buy an Eid sacrifice for her neighbor, but the neighbor moved to an unknown location and the woman was unable to find her?

If a woman exerted her effort in searching for the woman to whom the sacrifice was vowed this year, and did not find her until the Eid al-Adha passed, then she is not obligated to offer the sacrifice to anyone else, as long as she had specified the woman for whom it was vowed. It suffices for her to offer an expiation for an oath (kaffarat yamin (definition)). This is based on the general rule that whoever makes a vow and is unable to fulfill it must offer an expiation for an oath, as the Prophet, peace and blessings be upon him, said: "The expiation for a vow is the expiation for an oath."