It's possible that Vos is doping. I don't think it's possible, however, that she isn't hugely talented regardless.
There have been a few doping cases in women's cycling - Jeanson, Bastianelli, Bubnenkova, Leleivyte to name four - but with the depth of field being what it is, if you're one of the few that can make enough money to get by and do things professionally in the sport you have a performance advantage without needing to dope. Nicole Cooke, for example, was a very outspoken anti-doper, and though her last two or three seasons may have been disappointing, she certainly put together a very good palmarès before that. Does this mean Cooke is certainly clean? Perhaps not. But she talks a much better talk than most.
Vos hasn't transformed or suddenly discovered hitherto unrecognised skills. Her transition from being mostly for flat races or puncheur events to somebody who could win the Giro Donne took place over 3 or 4 years, rather than waking up one day thinking "I reckon I can be a climber" and suddenly dropping Claudia Häusler, Mara Abbott and Emma Pooley. She still gets dropped on longer, tougher climbs by the likes of these, and Evelyn Stevens, who has really come on over the last 2 years, it's just that she quashes them on intermediate and hilly stages and due to the lack of funding in the sport there are precious few races with real long, difficult climbs for people to drop her on, and the 4-5km climbs of races like the Emakumeen Bira are well within her remit, while on stages like the Stelvio one in 2010 she loses minutes.
Of course, while Vos earning much more than most women's cyclists (at least in part due to bogarting the prize money) does mean that she can afford better training, better planning and can be more dedicated than most of her competition, it could also be argued that it does also mean that she can afford a more sophisticated doping program than those the likes of Leleivyte (first-generation EPO) and Bastianelli (dodgy dieting supplement) used. But because of the lack of depth in the women's péloton and how dominant she has been ever since childhood so no particular performances are really abnormal for her, it's not as easy to draw an obvious conclusion as it would be for somebody who replicated her preposterous level of achievement in the men's field with its far larger field and much greater extent of specialization.