I would prefer it if topics here stayed about GIT and not about VPN since this is not a forum about VPNs, but here is little a comparison between GIT and VPNs:
Similarities:
Both tunnel traffic from one LAN to another.
Differences.
VPN uses encryption to ensure privacy. GIT does not, to maximize speed.
VPN does not typically tunnel broadcast traffic. GIT can.
VPN does not tunnel IPX traffic. GIT can.
VPN tunnels all traffic. GIT can be configured to tunnel only selective traffic based on port numbers or unicast vs broadcast.
VPN forces private LAN addresses and helps configure a NAT-like environment. GIT does not - GIT can work without private IPs but will not help you set up a LAN if that is required to get what you want from GIT. Some situations may require you set up LANs the way VPN would result in order to best use GIT (same subnet and no IP conflicts accross all physical networks).