vos is not really formal in Argentinian Spanish, although it does share an etymologocal root with vuestro merced/Usted.
Argentinian Spanish is itself pretty different from other Latin American dialects, but in general vos is used interchangeably with tú, to the point where the accusative form of "vos" is also te - ie, "Vos te levantás" instead of "tú te levantas" (note the location of the accents in each example).
The connotations of formality (or lack thereof) are subtle and vary much more widely than you might imagine - far more than the comment limit on HN would permit - but by and large it's better to think of vos as the informal pronoun, and the only question is whether it's used alongside tú, in place of it, or in some weird combination that has its own subtle connotations.