[quote=DeathstrokeSW]
But how were they given those abilities?
[/quote]

Through the use of the Hogyoku which is something that allows the barriers between Shinigami and hollow break down, granting one the powers of the other. Shinigami exposed to this break down of the souls became Vizards. Hollow exposed to this break down of the souls become Arrancar. Obviously the two are not supposed to mix hence the Hogyoku. It can grant things the affected being could only dream of attaining. 

On record you'd think Vasto Lorde are stronger than Arrancar but it does seem difficult to tell seeing as Harribel went from being a Vasto Lorde to an Arrancar yet she wasn't even the strongest one. 

It's also unfair to count Ichigo's Vasto Lorde form as I believe that is something entirely different from the pure hollow Vasto Lorde. Also Ichigo is a rare case...many things factor in here and we cannot simply say because Ichigo attained a Vasto Lorde form that Vasto Lorde are stronger than Arrancar...to be honest, Why would Harribel ever agree to become an Arrancar or why would Aizen make her one if she was strong enough already? it seems to me that by turning hollow into Arrancar that they become stronger...perhaps not in base form but when releasing they become stronger, then when they go to second release (essentially a Hollow Bankai) they become outrageously strong...stronger than in Vasto Lorde...It's like a beefed up Vasto Lorde...

Annnnnnnnnnnnnnd I've typed to much shit that is kinda pointless O.o