Vita enters A's on a hunt with Zafila for strong Linker Cores to absorb into the Book of Darkness. She targets Nanoha Takamachi, which she deems to be a juicy target, unlike the usual TSAB "redshirts" which can barely fill up a single page of the Book of Darkness.

Nanoha puts up a decent fight, but Vita tilts the deck with two cartridges (bullet shaped things that contain high-pressure magic) manages to overpower her, obtaining a power superiority so vast that she heavily damaged the Raising Heart. However, before she could get the Linker Core, Fate Testarossa and Yūno Scrya come to the rescue. Vita fights them off with her fellow Wolkenritter, Signum and Shamal, and they declare themselves enemies. The battle ends with Shamal finally managing to get Nanoha's Linker Core and Nanoha breaking Vita's barrier.

Four years after the A's epilogue, Second Lieutenant Vita is sub-commander of the Stars Squad in the TSAB, working directly under Nanoha, who, in turn, obeys Hayate. She and the other Wolkenritter were kept close to Hayate and each other-- Shamal and Zafira are support, Signum is with the Lightning Squad-- because they work most effectively as a team.

(Source: Nanoha Wikia)