I suspect you can, but who knows, maybe it's a special tarif that requires automated refills. You know who knows? Vodafone knows. Call 'em up and ask, you should be able to even get English language assistance.
I use a Vodafone CallYa tarif and can top up with a "CallYa scratchcard" (funny, they still call it a "scratchcard"), but there are several different CallYa tarifs.