But when talking about her and whether her work is indeed valid we need to be discussing them. Their work in this area is the bar to which other work is to be compared and discussed.
If say a new painter was just discovered from 1860. Had a large unseen body of work that was heavily influenced by the Impressionists; Monet, Manet, Pissarro would all have to enter into to the discussion. Even though the new artists work would of course have to stand on its own, the bar already set by the established artists would be important in the discussion of influence and validity.
In Vivian case her work whether it is more than just snap shots as you say its important to see how it was influenced and relates to others working within the same art form. In this case other established street photographers. That also helps figure the actual importance of the work.
Vivian certainly was influenced by those I mentioned but her work also holds up on its own in the light of those others. Though they influenced her, she still speaks in her own strong voice.