As your device plan is a seperate agreement (technically) from your airtime plan, I would assume that you'd use the original loan agreement you were sent/signed as the evidence that these payments are for that purpose on any returns. If you call 191 (or get in touch on Social Media, as they don't have any account access through the forum) they might be able to send you a summary or something, but I honestly wouldn't be surprised if they say it's not possible.
I'm not sure if there's any better page out there, but https://www.vodafone.co.uk/terms-and-conditions/consumer/device-financing-definitions/ is what I'm using to get that
