[Vuejs]-Vuex: Store module cyclic dependency

I’d construct your b store’s val as a default, not including any info from settings. Then have a mutation or action that sets it up properly from settings (a). Probably should be an Action, since Mutations only get the local state, whereas Actions get a context that includes the rootState from which you can access the other modules.
Alternatively you can just use a getter for b.val; getters have access to rootState (and rootGetters) as well.