Vita is Grace's childhood best friend. They attended Rearden together and eventually became college roommates, so she's a frequent guest in nearly all of Grace's random flashback memories throughout the novel.
Vita was even with Grace the night she met Jonathan, though Grace notices her bestie gives Jonathan "only her best manners" (16.279), i.e. the super fake politeness she reserves for people she truly despises. Vita serves as the maid of honor in Grace and Jonathan's wedding, but then ghosts her completely. Despite not having seen Vita in nearly two decades, when Sylvia asks Grace if she has any close friends in the city, Grace immediately thinks of Vita.
Thankfully, readers do eventually meet the elusive Vita. When Grace is staying at the lake house, her father tells her that Vita contacted him when she read about Jonathan's nefarious ways in the newspaper. Grace works up the courage to plug the phone back in and track down Vita's office number, and the two old friends meet up in person shortly after.
Grace is understandably super nervous to see her former bestie, but Vita tosses all the nerves out the window by warning "Gracie" that she's going to give her a hug. Soon they're chatting like no time has passed until Grace finally asks if they're not mad at each other anymore. Vita says she's mostly mad at herself, and we learn that Vita saw through Jonathan's superficial charm right away. When she went looking for Grace at the frat party and found her with Jonathan in the stairwell, she got a vibe from Jonathan that said You watch yourself. This is mine. Mega-creepy.
Collegiate Vita wrestles with whether to tell Grace about the neon warning signs she sees all around Jonathan's devilishly handsome face (emphasis on the devilish). After a few months of watching her best friend fall in love with a psychopath, Vita downs some liquid courage and attempts to talk to Grace about her concerns. Unfortunately, Grace is too busy writing "Drs. Jonathan and Grace Reinhart Sachs" all over her day planner to really listen to what Vita's saying. Vita realizes she can't control Grace's decisions, so she has to let her friend go and hope for the best.
In a not-so-subtle nod to etymology, "Vita" is Italian for "life." It's too serendipitous to be a coincidence that as Jonathan becomes a stronger presence in Grace's life, Vita starts to fade away, ultimately disappearing altogether. After the two old friends reconnect, Vita is also the one who helps Grace find a new therapy practice in Connecticut, essentially helping her reclaim her life without Jonathan.