I was able to use 1 VPN endpoint to access systems on different subnets which are on different VPCs. I followed this: https://docs.aws.amazon.com/vpn/latest/clientvpn-admin/scenario-peered.html
I updated the routes that were listed in the VPN endpoint.
Yes, the Client VPN endpoint is per VPC!. You can only associate the subnets in a particular VPC and not subnets from another VPC. When creating the Client VPN endpoint, remember you are asked to select a VPC ID and not VPC IDs. Say if there is a way to associate multiple VPC IDs to a single endpoint, then that would be where you can associate subnets from multiple VPC but this is not the case here.
"If you specified a VPC when you created the Client VPN endpoint, the subnet must be in the same VPC. If you haven't yet associated a VPC with the Client VPN endpoint, you can choose any subnet in any VPC.
All further subnet associations must be from the same VPC. To associate a subnet from a different VPC, you must first modify the Client VPN endpoint and change the VPC that's associated with it."
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