germānice vocative masculine singular of germānicus Germānus + -icus Germanic
it's not a vocative here. it's an adverb (germānicē). so it means: "We do not speak Germanly."
Yes, the -us, -a, -um adjectives make their adverb with the suffix -è (long e, I'm trying to express here). Whereas the vocative singular of a masc. adj. in -us is the short -e.