The same place as Van Gogh and Emily Dickinson. She had little influence while she was alive, but will gain more as time goes on. Posthumous fame is a cliche in the arts.
I'm only an amateur aficionado of photography, but Vivian Maier seems a bit different. The quality is undeniable but the style seems a bit derivative or antique by 2010 standards. At least so far, I don't see territory that hasn't been well covered by other street photographers.
Maybe more treasures will be unearthed from the collection, though.
A thought just struck me: maybe she just couldn't afford, or never had time, to get her negs developed. What sort of photographer might she have become if she could have closed that loop just a bit faster, or easier?