This lists 3 agents at Ralph M. Vicinanza Ltd.; Matthew Mahoney isn't listed (but, of course, that doesn't mean anything; Mr. Mahoney could be a junior agent or it might not have been updated in a while).
Apparently, the agency has an
Amazon page. This *might* be a listing of their stable of writers, but I could be completely wrong on that. (Might just be a listing of books people at the agency has read and liked.)
Holy moly! Just found this at
this site:
Ralph M. Vicinanza Ltd. is a New York City based literary agency that handles literary and commercial fiction and non-fiction, particularly specializing in fantasy, science fiction, and thrillers. Other areas of interest include science, history, serious spirituality, literary fiction, women’s fiction, and young adult fiction. Their authors include Stephen King and Robert A. Heinlein.
Yeow - King and Heinlein! Two biggies at this agency. (Still nothing about the agent the OP mentioned, though.)
Ah, there's
this on Matthew Mahoney on Query Tracker.
Lastly, there's Mr. Mahoney's
Agent Query page.
~Nancy