As veth interfaces can only exist in a pair of two, there is a pretty old issue from the time they got introduced when removing them.

Calling ip link add veth0 type veth peer name veth1 will add the interface pair (two new interfaces veth0 and veth1).

Removing one of them ip link del dev veth1 will immediately remove the pair interface from the Kernel.

This will NOT call the remove() helper from vyos.ifconfig.interfaces which will leave a DHCP(v6) client running for all time.

This issue was also worked around with altered smoketests https://github.com/vyos/vyos-1x/blob/ad60030a813e8913dbf639aa449081ce1c8287d6/smoketest/scripts/cli/test_interfaces_virtual-ethernet.py#L39-L60