is IPv6 for configuring an VIP address for loadbalancing supported with NSX-T 2.5.1?
I don't think it does.
2.4 introduces ipv6. 2.5 had some improvements on it.
3.0 has ipv6 vips VMware NSX-T Data Center 3.0 Release Notes
"IPv6 Support for LB Virtual Servers and Members - Load balancing IPv6 VIP to IPv6 members is supported."