The legacy "ip vrf XXX" mechanism only supported IPv4 - you did not need to specify which address family it was for due to this reason. The newer VRF syntax allows you to select which address family you want to use it for - by default no address families are activated. If the interface is running v6, and you need that v6 link to belong to a vrf, then you need to ensure that the vrf has the address family activated.
You will get errors such as these if the address family is not first activated:
vrf definition TEST
interface loop 100
ip add 126.96.36.199 255.255.255.255
%Loopback100 is linked to a VRF. Enable IPv4 on that VRF first.
ipv6 add 2001::/128
%Loopback100 is linked to a VRF. Enable IPv6 on that VRF first.
Hope this helps