i think you need to go back to CISAS for them to resolve it but am not 100% sure - will ask @Andrew-G who i think will have a better idea than my guess
CISAS can give VM a poke with a sharp stick, and although they can't technically enforce an adjudication, they've got some scope, such as threatening to suspend VM's membership of the CISAS scheme - see 4.6 in the CISAS scheme rules. So that's your starting point - report non-compliance to CISAS. Under Ofcom's General Conditions, para 4.3b requires that telcos comply with an adjudication. Failure to do so means they are in breach of the General Conditions, and Ofcom can apply the sanctions defined in the Communications Act 2003. And this isn't a first time for the clowns who run VM - in March 2021 they were given a formal warning by Ofcom for non-compliance with CISAS adjudications.
I'd also recommend that complaint is lodged with Ofcom about VM's poor handling of complaints and subsequent failure to comply with an adjudication. Ofcom still won't get involved in the detail of your complaint, but it'll at least give their CEO something to talk about next time he visits Ofcom other than his dreadful 1970s haircut.