"Hello, Livia and Corinna!"
Translation:Salvete, Livia et Corinna!
Don't think of Livia as a feminine word, but a first declension, an a stem. The 1st declension is mostly feminine, with a handful of masculine words. The second has a few feminine word you'll encounter. The third has many feminine words and they follow the same patterns as masculines in that declension.
The only instances I can recall for the vocative being different than the nominative is for masculine second declension nouns. When they end in -us in the nominative singular they become -e in the vocative singular. When they end in -ius in the nominative singular they become -i in the vocative singular.
There are masculine and neuter nouns that would also experience no change, such as nauta (masculine, means sailor), and donum (neuter, means gift).