[name_f]Vi[/name_f], [name_f]Letty[/name_f], and [name_f]Etta[/name_f] are the only ones I can think of off the top of my head. I love both [name_f]Letty[/name_f] and [name_f]Etta[/name_f], and [name_f]Vi[/name_f] is a great one just to call her by occasionally, while still introducing her/referring to her as [name_f]Violetta[/name_f].
[name_f]Via[/name_f], [name_f]Ettie[/name_f] and [name_f]Lettie[/name_f] are my favourites of your choices. Personally I find Vio to be a lovely nickname too.