I would understand if she was in the beginning, and the end but once Vincent killed Zasa's guys she wasn't even heard of again even Sonny used Lucy Mancini a couple times. Maybe she was just used to show that Vinny was a ladies man like Sonny, but if you are gonna do that use a not known actress who won't cost money.

"Well you just do what I tell you to do! Goddamn it! If I had a wartime consiglieri -- a
Sicilian -- I wouldn't be in this shape! Pop had Genco -- look what I got." - Sonny Corleone