Issue with IPsec Connection to FortiGate Behind NAT - vyOS 1.4 (set vpn ipsec nat-traversal 'enable' not available)

General questions
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3

It looks like you’ve got some sort of auth problem.
But for a site behind NAT you should try

set vpn ipsec site-to-site peer FORTIGATE force-udp-encapsulation

“force encapsulation of ESP into UDP datagrams. Useful in case if between local and remote side is firewall or NAT, which not allows passing plain ESP packets between them”

I’m not much of an IPSEC Expert, so I can’t really provide you with much other useful feedback/answers I’m sorry.