First mistake, Two different dungeons. Correct me if i'm wrong but vault is quite healer intensive to begin with. espescially compared with Dusk Vigil...
Second of all, you're comparing two different tanks. You should compare the same tank(You can use your fc) And use both full str & full vit.

Difference is unreal for most dungeons/trials/raids its not even needed to use ANY vit what so ever.