yes, Andrus, your vocative is correct, along with your entire 2nd declension paradigm. There is indeed a (helpful) distinction between the genitive singular and nominative plural if you choose to use the contracted genitive.
-i genitive is more common during the Ciceronian era whereas the contracted -ii genitive is more common during the Augustan era. It does not really matter which one you choose to use, but I think most modern textbooks prefer the -ii for its simplicity and regularity. The accent for the contracted version falls on the same syllable as if it were not contracted.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae