Router device, that can connect to internet, establish VPN using internet and NAT packets selectively

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http://serverfault.com – I have several shop networks and office network. Those networks are joined using simple PPTP VPN server. When one machine pings another from another network, I need the source address of the packet to be the IP address of the source machine, not IP address of the router, that connects to VPN. So, I have simple routers in the shops, that can connect to internet (L2TP or PPTP) and can connect to VPN using that internet connection. The problem is, that those routers NAT all packets both to internet and to office network. And I need packets for office network to be skipped from NAT. Do you know (HowTos)