Is what I said to myself—that I would name my pharmacy "Sabeel Allah" (Path of God) if I succeed, graduate, and open it—considered a vow, and must I fulfill it, and would I be sinful if I do not do so?

A vow (nadhr) is not binding unless it is made with a clear formulation that expresses commitment. What you thought to yourself is not considered a vow. Even if the formulation existed, you are not obligated to fulfill it, because it is not an act of worship (qurbah). There is no sin upon you for not naming the pharmacy "Sabeel Allah" (Path of God), and making vows is disliked (makrooh).