It would help if you told us what the VPN protocol / settings / software exactly is. Different VPN setups have different requirements for the way you have to configure the networking on the systems being used.
In general some VPNs pass through NAT, others do not do so without setting a configuration option to allow NAT transversal. Some have the choice of using an IP protocol like AH/ESP on the wire, others allow encapsulation in UDP packets.
In general if you enable UDP packet encapsulation, NAT transversal, and configure the MTU of the VPN packets to be small enough that they can be further encapsulated or have further headers added to them and still fit in an 1500 byte ethernet frame or less, things tend to work better.
There is no GENERAL reason you can't run a VPN inside a VPN inside a VPN ... as much as you want. Some VPN software tries to lock down other networking ports / software on the PC it runs on other than its own as a security measure, but that is not VPN related, that is just something that may be done by certain software providers unrelated to the VPN technology.
Running a VPN may change your routing table so you may need to be aware of how your routes are defined if you have a particularly complex setup.
You cannot have multiple identical MAC addresses on the same LAN segment, but it may work OK on different LAN segments. It is commonly a feature of wireless or wired routers to be able to change their MAC address to match the PC behind them or to assume a specified MAC address. If you're worried about / having problems with doing that and not changing your PC MAC address too then just change the PC MAC (often also a configuration option in the PC's driver) and you're all set. Or get one of the $6 USB to 10/100 Ethernet dongles and plug that into the router when the router is set to the PC's MAC address. Personally I assume it'll "just work" since they program the routers with MAC cloning to handle this case....
If you had (Laptop) --- (PC) --- Net Jack
then you can either BRIDGE the PC's two ethernet ports in which case the Laptop would look like it is connected directly to the Net Jack even though it goes "through" the PC's bridging. The LT's MAC would be directly on Net Jack's network, and the LT would need its own IP address assignment etc. If that was all OK, however, the VPN or whatever it ran should work just as well as if it went straight into the Net Jack ignoring the PC's presence.
Verify that any firewall software on the PC isn't blocking the packets / protocols you need for the VPN.
Laptop -- PC -- LAN
If you use the PC as a NAT gateway for the laptop to connect through like with an Internet Connection Sharing setup, the PC and the Laptop will have identical IPv4 addresses as is seen by the external network LAN the PC is connected to. The port number would be the main way the PC would know to forward certain traffic to the Laptop instead of processing it itself. Ensure the PC firewall and NAT sofrware is set up to pass the protocols / ports / traffic types your VPN needs. Use UDP encapsulation and NAT traversal settings for the VPN.
Personally I'd buy a decent router and use that in front of your PC and Laptop and get one that has the right kind of VPN pass through, port forwarding, firewall, MAC cloning, etc. support you need for the VPN setup.
