Vi, Diana, Jinx, and Thresh are the only champions to feature lyrics in their login themes.
Vi is the shortest in-game champion name (only two letter long), the Roman 6 (Officer has the number tattooed on her left breast and emblazoned on her cap), as well as ablative singular of Latin vis "force".
Vi is a Zaunite sumpsnipe who went on to become a Piltover warden for unknown reasons.
Vi and Jinx are often implied to be sisters. The first reference to this was on Jinx's release with one of her quotes, "You think I'm crazy? You should see my sister", but this sister could very well be a figment of her crazed imagination and not a real person. In any event, there was never any concrete evidence that the two were siblings, but players equate Jinx's antagonization of Vi specifically as "sisterly bonding".
Incidentally, Jinx only antagonized Vi specifically on English-speaking serves. Other servers showed her antagonizing other champions, such as Lucian.
Shopkeeper Lyte later says to Vi, "You look like your sister! Er, wait, I'm not supposed to talk about that... ". This could be a confirmation that Vi has a sister, but could also be an example of Lyte repeating rumours he's heard from players (such as the rumour he heard from players that Lux and Ezreal were dating, which Lux herself later denied after hearing the rumour herself).
There is a significant indication that the pair did grow up together, however. With Vi currently suffering from amnesia, Jinx has taken it upon herself to try and trigger her memories.
In an interview with Ghostcrawler, he mentioned that Jinx and Vi are "sisters", but later confirmed he was merely stating what he thought was true, and that the narrative team said otherwise.
Vi remembers nothing from her early childhood, not even her real name ('Vi' comes from the tattoo on her cheek)
Ekko remembers Vi from before she left for Piltover.