I have this "old" setup that's working as wanted.. openvpn client connected with danted with polipo proxy server, nextcloud file server and apache server on the same machine. This setup do what I want it to do, is it possible to do with netplan?? Setup below.
Also a the tun interface will added with openvpn
auto lo eth0 eth0:1 iface lo inet loopback
iface eth0 inet dhcp
up ip route add 192.168.71.0/24 via 192.168.1.5
iface eth0:1 inet static address 192.168.1.15 netmask 255.255.255.0 broadcast 192.168.1.255 network 192.168.1.0
post-up ip route add 192.168.1.0/24 dev eth0 src 192.168.1.15 table admin post-up ip route add default via 192.168.1.1 dev eth0 table admin post-up ip rule add from 192.168.1.15/32 table admin post-up ip rule add to 192.168.1.15/32 table admin post-down ip rule delete from 192.168.1.15/32 table admin post-down ip rule delete to 192.168.1.15/32 table admin
ip rule 0: from all lookup local 32764: from all to 192.168.1.15 lookup admin 32765: from 192.168.1.15 lookup admin 32766: from all lookup main 32767: from all lookup default