@Mark Allison Thank you for reaching out to us. As I understand you are trying to access key vault over private link using IPv6.
As per this article Only IPv4 addresses are supported at this time.
You can share your feedback here https://feedback.azure.com/d365community/search/?q=Ipf6+key+vault+private+link which is closely monitored by our product group team.
Let me know if you have any further questions.