Are you certain?
And if they weren't his pants, whose were they? Why does he pick them up in the same manner as if he was about to put them on? Why is it that he is delayed getting to the church, as if he had stopped to get dressed first?
ETA - You know, I think you may be right. Looking at a vidcap of one scene I can see the blue (looks like denim) clothing slung over the chair Vincent is sitting on. And that was taken in the daylight, so either he had his pants off for an awful long time (which I think is contradicted by other shots of Vincent in that sequence) or it is another article of clothing or something.