First of all is this a Router/Firewall to Router/Firewall VPN or just a pc to a server VPN?
For the first most routers nowadays come with some sort of setting to set this up. Just plug in ip addresses/hostnames and a password or cert and youre a for an apple. No need to forward ports or anything. You will need different subnets on each end otherwise routing gets confused.
If you want to connect to a VPN server using the windows VPN connection you'll need a server with the correct ports forwarded to it from the router. For PPTP it will be 1723. Again you need seperate subnets so things don't get confused...
So for option one you need two vpn end points which will be connected pretty much all the time and for option two you need a server and a pc or whatever.
There are a couple of proprietry apps as well that Cisco uses where you need a client for it. An open source app such as OpenVPN is also available.
Far as I know with Hamachi its only the initial login to their mediation servers that goes through them. Once you logged in you directly connected to the other people in your groups.
Just don't like Hamachi because it tends to be a bit slow at times for some applications. Plus it's somewhat limited in that its a one to one link only, you cant say link an entire branch office to head office.
Its GRE port 47, and majority of routers dont have an option for it. If you have the option for PPTP-VPN under your standard applications then it automatically forwards it accordingly. Failing that just specify the server as a DMZ.
GRE is a layer 4 protocol. and its protocol number is 47. Like UDP is 17 and TCP is 6, ICMP is 1 and OSPF is 89. GRE doesn't have ports like TCP and UDP.