I'm not sure why you would want to use those guns honestly, the L1 lvl 91 guns are the 16" triples.
While it could be that the vanguard guns are unavailable for the L1 (which I dont know having never owned one), I doubt that. Did you check you didn't simply miss them in the list of guns?
EDIT: Test server L1 couldnt mount vanny guns either. Must be restricted.