MF,
In order to install offline vault, the client needs to be in cached mode at least once (so it can create an ost file). Once that's done, you can do the primary sync with offline vault, and it should be okay in either cached or non-cached thereafter. It's important to note as well, that you won't get the offline vault accesses unless outlook is officially "offline". Otherwise it'll try to hit the EV server regardless of cache mode.
micah