http://i2pforum.i2p/viewtopic.php?p=3168
(i haven't actually tried myself, because i haven't used vpn since switching to linux, but this seems like it should work) That is the rough plan, please consult the man pages [ip-link(8), ip-netns(8), etc.] for details. If you wanted to run everything except I2P through the VPN, you would need to attach everything (or at least the programs that are internet-connected) except the I2P process to the netns.