Hello laydit
Thank you for taking the time to contact us in regards to your vouchers as part of your order. We appreciate you raising this via the forums and welcome to the community.
As you have advised in your post, the information given when you spoke to the team is correct. The offer of the voucher would be from the third party, in this case, the site used to sign up for Virgin Media.
We understand this may have been some time ago, we would advise to check any records you have in relation to the broadband. More details on our 'gifts' can be found here virg.in/gifthelp. We appreciate this may not be the answer you were looking for but hope it helps you in resolving the issue.
Rob