Communication between VPCs and a transit VPC hosting VPN Server without Masking source IP
VPC peering does not support transit traffic.

Unsupported VPC Peering Configurations described several scenarious that are not supported.

This case is not specifically mentioned, but is implicit from the other limitations.  

Note also that it is impossible to fully configure the setup you are attempting, because when you route a given destination towards a remote VPC via a peering connection, there is no route table in the target VPC that is applicable to that traffic. Only the instance subnets are available.  Route tables apply only to subnets, and the default route table only applies to subnets without an explicitly assigned route table -- not to incoming traffic from a peering connection.  There is no way to route this traffic back to your instance.

This is why Hardware VPN, Direct Connect, and NAT devices are not available across peering, 

A Transit VPC is not what you have described.  These use AWS VPC Hardware VPN connections on the spokes and an EC2-based router at the hub to terminate the tunnels... and the transit routing between the tunnels is done within the instance -- not by the VPC network infrastructure.