The proper configuration is to have one link prefix assigned to the physical network interface on the VPN server and have a different prefix routed to the server from which you allocate a link prefix for the virtual network interface used by the VPN software. Unless you have specific reasons to do otherwise, you should make each of the link prefixes a /64. The routed could be a /64 or shorter.
Of those three prefixes involved you only mention that one is a /112 without mentioning which one of them it is. This makes me think you might be trying to use one single /112 for different purposes without even realizing what each of those different purposes is.
There are hacks one can use to turn part of a link prefix into a routed prefix, those hacks do however waste capacity on the router and requires better understanding of the protocols you are dealing with, so my recommendation is to use a real routed prefix.
If it is completely impossible for you to get a routed prefix either from your current provider or by switching to another provider, you can proxy neighbor discovery between the physical and virtual interfaces. ndppd promises to be able to do that. I haven't tested that particular tool, but I know from previous experiments that it is possible to do it.