Yes to treating the vasodilation - a good thing. And I do believe it's really helpful for some people.
My experience has been that coffee has several times given me temporary relief, but often has left me with more intractable migraines than I get without the coffee. I think there might also be a component of speeding up metabolism temporarily and then leaving it more sluggish than it was previously when it wears off. Possibly some extra stress response gets mixed in in some circumstances, and maybe it also gives an extra job to the liver or some other system that gets a bit overloaded. I'm speculating. But I know more than one other person who has told me that they get headaches if the drink too much coffee, and not if they stay away from it or keep it to smaller amounts. I know that's not everyone, just some people. It's only relevant if it actually works like that for a particular person, I'm not suggesting everyone or all migraineurs should avoid it. It's also possible that if I changed other factors for myself I would get different effects.
I've never heard of it till I saw you mention it in an other thread recently. Thanks for the tip. I'll take a look and see if it's available locally.
I haven't looked into safety or mechanism, just saw some interesting articles recently on choto-san.