a VPN often times FORCES all traffic to travel across the VPN connection. This is a function of both VPN client preferences, AND whether the VPN server allows the client to make connections that do not travel across the VPN.
If all your traffic is being forced across the VPN, then it is a question of whether the destination system is accessable from the VPN server.
For example, the company I work for forces all traffic across the Cisco VPN when I have it active. As long as the VPN is active, all my traffic looks like it is originating from inside the corporate firewall which restricts what I can and cannot access.
If I want to browse the web while the VPN is active, I have to configure my System Preferences -> Network -> Advanced -> Proxy to use the company's proxy server. I cannot make ssh, ftp, afp, smb, cifs, vnc, etc... connections outside of the corporate filewall while the VPN is active, this includes my home network printer, my home file server, my wife's MacBook, etc... I can only access stuff inside of work, and I can access the web ONLY if I setup a Proxy in my network preferences.
NOTE: Some companies configure their VPN servers to allow local connections to home servers, computers, and networked printers. Many do not.