The *A lounges at VIE are operated by OS rather than LH. Arriving from FRA, you'll have to clear passport control at FRA to enter then Schengen zone, so you'll arrive in the Schengen area at VIE. However, you'll depart from the non-Schengen zone to DBV, so you'll have to clear passport control again at VIE (this is usually before the gate, but, just to add confusion, some non-Schengen flights depart from the Schengen pier with passport control at the gate).

As C passengers, you'll all have access to the Business lounges, but the Senator lounges are only accessible to *G plus one guest (I can't remember if 1P is *S or *G).

There is a Senator lounge on C pier, a Business lounge in the shopping area, and smaller Senator and Business lounges after passport control on the A pier. The C pier Senator lounge is the nicest (but watch your timing if you depart to DBV via the A pier, and need to clear passport control).

If only one of you is *G, but you're all C pax, then you can politely ask if you could all get into the Senator lounge, but don't be too disappointed if you're not. You can all get into the Business lounges. I don't know about the shower situation in those lounges, as I only ever fly short-haul from VIE.