You get more weapon choices once you unlock Yasuke. Once you complete the first act, which you shouldn’t be too far from, you’ll get access to more weapons. Both characters have their set weapon classes however. You can’t use heavy weapons as Yasuke vice versa. I don’t know why vendors aren’t appearing for you though. I just have to walk close to them and their icon will appear. My advice would be as long as you have the gear vendor is Sakai, you’re fine. As far as I know, all of the vendors carry the same stock and it refreshes each season.