I’m digging into this a bit deeper. Recent question is, if VyOS 1.3 supports source-specific multicast for IGMPv3?
I’ve already taken a look at Firewall — VyOS 1.4.x (sagitta) documentation
And I’m not a 100% sure if set firewall ip-src-route [enable | disable] is the correct setting but I THINK I’m on the right track now. 
Or is there a possibilty to enable the netfilter IP options parameter manually?