I want to route everything comming from the local ip 10.0.8.129 with destination internet via a VPN Interface. I tried to add 10.0.8.129/32 and Outgoing Interface VPN under Network -> Routing -> IPv4 Rules but that is somehow ignored and traffic from that IP uses still the default route.
In your case you have not filled in the rule properly. You need to specify Incoming Interface, Source IP, and the Table. Then in static IPv4 Routes you need to add a default route and any local routes for that Table.
Thanks Lynx, your answer did work I just was confused for a long time that everything would stop working as soon as I added the rules but that was because with the custom routing table intervlan routing does break if you don't exempt local IPs.