Hi everyone!

Jool offers a stateful (destination) NAT64 variant (called BIB), which appears to not be exposed via VyOS config yet. This would allow to do IPv4 -> IPv6 translation similar to classical NAPT44. Following their docs, all it takes to create an entry is a single command specifying the IPv4 address and port, IPv6 address and port and the protocol. To keep it similar to the existing CLI, I'd propose something along this:

set nat64 destination rule 1 protocol [tcp|udp|icmp]
set nat64 destination rule 1 source address 192.0.2.1
set nat64 destination rule 1 source port 80
set nat64 destination rule 1 translation address 2001:db8:12:34::1
set nat64 destination rule 1 translation port 80
...

Best regards,
Michael