It is certainly not an oath. An oath is an invocation of the Divine Name in testimony to some truth, such as in court. Since you are talking to God, testimony to truth is not really an issue… Yes it is possible to lie to God, but that is not what is at stake here.
A vow is a promise to God to do a better and possible good. This would involve some deliberation… Spontaneous promises are not vows.
Additionally, depending on the circumstances of your average consumption, promising not to drink would be promising not to sin. If what you are promising to forego is something sinful (as it probably is, but not necessarily) then it is not a vow. Vows are for “better goods,” not “goods.”
Whether or not some particular instance of underage consumption is not the question, but it is involved, as I hope you can see.
Instead of making “pseudo-vows,” try making resolutions instead.
OATH. The invocation of God’s name to bear witness to the truth. A person, being conscious of his or her own fallibility, professes by an oath that God is omniscient and the omnipotent avenger of falsehood. For an oath to be licit, the statement sworn to must be true; there must be sufficient reason for swearing, i.e., regarding some matter of importance or because the circumstances demand an oath, as in a court of law; and the statement itself must not be sinful, e.g., not disclosing a secret that should not be revealed.
Oaths are assertive when God is invoked as witness to the truth of a past or present event, e.g., that a crime was not committed. They are promissory when God is invoked to bear witness not only to a future act but also to a person’s present intention of doing or omitting something, e.g., the promise to fulfill the duties of one’s office.
Oaths may also be distinguished as invocatory and imprecatory. They are invocatory when God is simply called upon as witness to the truth; they are imprecatory when, in addition, he is also invoked as the avenger of falsehood. In the Old Testament oaths of imprecation were very frequent, e.g., in the expression, “The Lord do so to me and more also.” In current usage they now occur in the familiar form, “So help me God.” (Etym. Greek oetos, a going; fate.)
**VOW. **A free, deliberate promise made to God to do something that is good and that is more pleasing to God than its omission would be. The one vowing must realize that a special sin is committed by violating the promise. A vow binds under pain of sin (grave or slight) according to the intention of the one taking the vow. If one vows with regard to grave matter, one is presumed to intend to bind oneself under pain of serious sin. Vows enhance the moral value of human actions on several counts. They unite the soul to God by a new bond of religion, and so the acts included under the vow become also acts of religion. Hence they are more meritorious. By taking a vow, a person surrenders to God the moral freedom of acting otherwise, like the one who not only gives at times the fruit of the tree, but gives up the tree itself. And vows forestall human weakness, since they do not leave matters to the indecision or caprice of the moment. Their very purpose is to invoke divine grace to sustain one’s resolution until the vow expires or, in the case of perpetual vows, even until death. (Etym. Latin vovere, to pledge, promise.)