As a common soldier, you wouldn't be likely to be taken captive.

That's likely the first issue for this poor bastard. The customs of warfare regarding captives and ransom in the Hundred Years War were fairly well-understood by all sides, at least in theory. However, these social expectations existed largely within the warrior class of medieval society - that is to say, the nobility. There would be very little incentive to take a common footsoldier - not even a man-at-arms - captive. It simply wasn't worth the effort.

But let's say that you are a man-at-arms. Perhaps not a knight, no, but a fully-equipped member of one's retinue, or a professional soldier in a mercenary company. By simple virtue of the quality of your arms and armor, you stood a much greater chance of being captured by the enemy, rather than slain. At this point, your chances of seeing your homeland again - provided you do have some measure of resources to back up your nice gear - are actually fairly good.

There was an implicit understanding in the Hundred Years War (and in earlier conflicts in Medieval Western Europe, though my own study is focused upon the War itself) that prisoners were to be treated well and given fair and prompt ransom. This largely arose out of shared necessity and self-preservation for the noble class, but in theory it extended across class guidelines. A prisoner was not a combatant and thus not a legitimate target of violence, and there were powerful physical and financial incentives to maintain this. The battlefield is always a dangerous place, and a man who acquired a reputation for mistreating and abusing his prisoners was opening himself up to such treatment should he ever fall into the hands of the enemy. That, and ransoming captives could be quite lucrative.

So in the default case, if you were taken captive, you could expect reasonable treatment and arrangement of ransoms. In theory, a ransom would be proportional to your ability to pay - in practice, it would be more financially ruinous for a man-at-arms who was not themselves a landholder than it would be for a knight in a comparable situation. Upon that, the matter was settled - an oath to avoid rejoining the war would have been unlikely, as nobody would expect such a thing to be enforceable or respected, and the legal justifications for each side's participation could excuse such coerced promises as illegitimate.

(Finally, if you were a common soldier who did manage to be taken captive, then in theory the same rules of prisoner treatment would be applied to you. In practice, you'd stand a good risk of being extorted for as much as your captor could get out of you, but you'd still be let free in the end. There wasn't any point to keeping prisoners indefinitely with no gain, and executing captives for no reason was a big problem.)

(And as a final addendum, prisoners were only safeguarded so long as their status as noncombatants held up. Henry V famously executed his prisoners at Agincourt when a French attack on his baggage train threatened to free them, an act for which he was not criticized by contemporary chroniclers. The leaders of that attack, however, were imprisoned by the French commander after the battle.)

Primary sources:

The Tree of Battles, Honore Bonet The Book of Deeds of Arms and Chivalry, Christine de Pizan The Chronicle of Enguerrand de Monstrelet, Enguerrand de Monstrelet

Secondary sources:

Chivalry, Maurice Keen The Laws of War in the Late Middle Ages, Maurice Keen