I am takin this to mean that the guns listed will be the only mounted options. Not simply added ontop of the Vickers.
I don't think so. "1 x Lewis (single overwing)" is listed as being added to the Spad S.VII, Albatros D.III, Albatros D.Va, RE8, F2B and Breguet yet I'll bet you none of these are loosing their Vickers/Spandau. I can't imagine any German pilot removing both MG08/15's from his Albatros just to fit one lewis.
Similarly, adding a extra lewis to the Spad S.VII was not unknown as it gave it two guns but I've never heard of a Lewis only Spad S.VII so when ROF list "1 x Lewis (single overwing)", I'm pretty sure they mean his,
So with the three weapons mods, we'll end up with four armament versions. Here's what I think they'll be...
"1 x Lewis (single overwing)"
1 Vickers and 1 Extra Lewis gun , as per Charles Nungessers personal Nieuport
"2 x Lewis (double overwing)"
1 Vickers and 2 Extra Lewis guns
1 Vickers and 8 LePrieur Rockets
BUT this is what I would like, the RFC Nieuport with only 1 Lewis gun and no Vickers, like Bishops famous machine B1566