Received a text from vodafone offering a £10 value bundle at half the usual price. I topped up with £5 and sent HALFBVB to 49500, as per the opt-in instructions....got a reply saying i wasnt eligible for this offer....?
Provided the offer text was sent to the same phone number that you replied from, and provided you had an adequate balance, I can see no reason why you shouldn't qualify for the offer.
Unfortunately I can't find any additional Ts&Cs for the offer, as it isn't covered by the general BVB terms, and I can't spot it amongst the consolidated Ts&Cs, although things can hide in very unexpected places. However, if they aren't made public, presumably there aren't any.
So I'd suggest that you seek help on this from Live Chat or CS - see the Contact Us page.