Vincent wears a leather shirt in advent children and a vest in Dirge of Cerberus. It's actually kind of hard to see as he's wearing his capelet and cloak the whole movie.. But it is there anyway. Exactly what it looks like... Well noone knows! You can see some parts of it but never a good shot of what his vest actually looks like, only small glimpses of it and usually so much motion blurr that you can't make out much details.
I did, however, find a picture of a Vincent model on the internet.
This picture is perhaps not the official Advent Children version but it's based on the changes Vincent went through from the game compared to the movie, and as I don't know what Vincents vest/shirt looks like I'll make something inbetween, following the hints from the movie but the guidelines from the model above.
There is a short collar, 5-6cm tall with a belt attached (visible in the movie). The two sides of the front is overlapping atleast ~10-15cm and attached with 4 belts (marked in orange below). While the arms looks like they are made in other material on the model above, it looks more like it's 2 sleeves attached to the vest in the Advent Children movie so I'll probably go with that.. But have not decided yet.
This is an idea of the design so far, made out of an old bedsheet I found lying around. It's supposed to be a tight fitting vest which goes further down than the waist (marked in purple), but stuffed into the pants carefully. There is a lot to adjust on it but I've gotten most of the meassurements and parts of the pattern now. The colored lines are for references of stuff I need to keep in mind, like the belts, how to attach, which part belongs where and so on. Such a mess, but atleast it shows the main points and problems. The front is split in to left (light blue) and right (red).
The collar (dark blue) is yet to be designed as attaching it is the catch.. I'll most likely have to make a test-piece and attach with rough basting so I can try it a few times before I do it properly. Same with the sleeves. I'll take careful meassurements on sleeves and collar.. Actually.. I'm thinking of cheating and not add a collar with belt as it's really annoying and noone will see it. But I might just do it.. Hard to say.
The good thing with making this vest compared to cheating and taking a thin black sweater is that leather is much more stiff and firm. I'll be able to properly attach the shoulder and make the cloak attachable with buttons (light green and dark green marks).. That's my plan anyway!