What is the ruling on working in the civil engineering profession after having made a "vow" not to practice it, whether this vow was made to Allah or by an oath, and do the earnings from this work become unlawful if this "vow" is not expiated for?

You are not bound by the vow as long as you did not utter it, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for what they whisper to themselves, as long as they do not act upon it or speak of it." Doubt about uttering the vow does not obligate you. If you swore not to do something and then did it, you are required to offer one expiation for an oath, due to the repetition of the oath for emphasis. The money earned is permissible, and the expiation remains your responsibility. We advise you not to preoccupy yourself with what has not happened, and to beware of obsessive thoughts that might lead you to seek help from a psychiatrist.