They would have made vinegar, but when an armed cohort wanders in, thirsty from the field, best approach is to serve the freakin’ wine. :’)
Vinegar was one of the barter items Roman soldiers rec’d as pay. The other was salt.
I was just wondering if they actually could distinguish between a vessel that had contained wine and one that had contained wine vinegar. I would not be surprised if that is possible but it sounds like a difficult task.