Just wanted to make sure you knew that if you fodder +s into another weapon, it'll be added to the +s of that weapon. So if you fodder a +1 R you got from gemcha into your phantom/soul/whatever weapon that is +2, your phantom/soul/whatever will become +3 afterwards. The max is +99.
20% according to the JP wiki if you have her leveled.As for Erebus that does sound rather good, so long as the other team members cover the essentials already. The Kamihime wiki doesn't seem to have %s on the Vigor buff unfortunately.