Multicast IPv4 and IPv6 addresses are never supposed to be assigned to interfaces — they are managed by joining multicast groups.
However, Linux kernel doesn't care, and VyOS CLI doesn't prevent that either:
vyos@vyos# set interfaces dummy dum0 address 224.0.0.50/24
[edit]
vyos@vyos# commit
[edit]
vyos@vyos# ip addr sh dum0
11: dum0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
link/ether 72:be:a6:10:99:8f brd ff:ff:ff:ff:ff:ff
inet 224.0.0.50/24 brd 224.0.0.255 scope global dum0
valid_lft forever preferred_lft forever
inet6 fe80::70be:a6ff:fe10:998f/64 scope link
valid_lft forever preferred_lft forever