I personally don't think Vi is a Lesbian, nor Bisexual. Being a badass does not turn a woman into a Lesbian or the such, as being a well mannered and sensitive male does not make him Gay. Tying behavior and sexuality may seem reasonable in most cases, but it is not, at least, not Behavior in it's full extent, because some attitudes could be labeled as "Proper and Usual in Homosexuals" (whether it is accurate or not to do so), but most attitudes are specific to a person.
Thus, tying a general condition (Sexuality) to a specific condition (Behavior) will result, in a lot of cases, in a misjudgement, as most of the criteria used in the making of that judgement is specific to a person and not relatable to his or her Sexuality.
To sum up, Vi is not Lesbian nor Bisexual, at least, not judging by her Behavior. I think she is probably straight, considering she has never shown any particular interest in Caitlyn, aside, of course, of her friendly, ironical banter, but that is a far fetched assumption on my part, and only my particular judgement.
