While digging around in vyatta-bgp.pl because of T1301 I noticed that vyos allows for separate peer-group within the ipv6-unicast family compared to the default peer-group for the neighbour.

For example, config settings that are checked:
https://github.com/vyos/vyatta-cfg-quagga/blob/current/scripts/bgp/vyatta-bgp.pl#L304

There is also a whole bunch of code related to looking for peergroup6:

https://github.com/vyos/vyatta-cfg-quagga/blob/current/scripts/bgp/vyatta-bgp.pl#L1136
https://github.com/vyos/vyatta-cfg-quagga/blob/current/scripts/bgp/vyatta-bgp.pl#L1184
https://github.com/vyos/vyatta-cfg-quagga/blob/current/scripts/bgp/vyatta-bgp.pl#L1434
https://github.com/vyos/vyatta-cfg-quagga/blob/current/scripts/bgp/vyatta-bgp.pl#L1445

Since moving to FRR, this is no longer required as FRR considers peer-group to be a top-level setting for the peer, rather than having it redefined-again under the ipv6 address-family.

Looks like you used to need to set it in again under the address-family, but this was removed in FRR here: https://github.com/FRRouting/frr/commit/c8560b44e042851aaa4a60b6b9f92a3d3500c371