The reader is supposed, initially, to think that this unknown bartender is Stratford, but the story unfolds to reveal that it was in fact Willikins. Also, think about it. By that time, Vimes knew what Stratford looked like and it would have had to be something of a disguise to deceive Vimes, and that is beyond Stratford's capabilities - he hasn't got that kind of subtlety. Also, during the dialogue after he was captured, Stratford would have mentioned he was the bartender if that had been the case, not just that he'd observed Vimes drinking the cocktail, e.g. something along the lines of 'and I gave you three
of those cocktails!' TP would have given that much of a clue at the very least if he'd intended it to be Stratford.
If you still really doubt it, you could always ask the author!