I haven't done a routed mode configuration, so this is my guesses. The LAN IP of the VMX must match the address assigned by Amazon. This would be compulsory. Your VPC would need a return route pointing to the VMX LAN IP. If the VMX needs to communicate to subnets outside of the current VPC subnet it is in, you would need to add a static route on the VMX pointing to the VPC default gateway for that subnet. An easy way to do this is to check another EC2 instance in the same subnet and see what it is using for its default gateway.
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