we can say they met outside of vash first! what was she doing around the time vash ghosted on her?
I'm getting a grandmaster but not as powerful (and without a planet) vibe from him. so basically he just views humans as entertainment and doesn't really care much for their backstories and what not. so maybe he met vash, then met rae, and when he saw them next and they were together he'd be like "oh great my two favorite people know each other!" (though mind you anyone who entertains him is his favorite person atm).