It should be fine if you connect the Mac OS via VPN and configure VirtualBox to use NAT as network type. The guest OS does not have to know about VPN.
Depending on the type of VPN, there is an option under Advanced in the network system preferences, to send every traffic via this VPN connection.

Update:
As I said, it depends on your type of VPN, but in general, follow this:

1. Get your VPN connection running on Mac OS. If available, select "Send all traffic over VPN connection": System Preferences → Network → (select your VPN connection on the left side) → Advanced...
   Advanced Settings http://img682.imageshack.us/img682/3091/1003220001.png

2. In VirtualBox, set Attach to.. for the network adapter to NAT.

As all traffic is now sent over your VPN, the traffic generated by VirtualBox is sent via your VPN. Of course you cannot see any servers or whatever inside your guest OS as it is behind the NAT but you should be able to connect to servers, etc. via IP or name.

It should be fine if you connect the Mac OS via VPN and configure VirtualBox to use NAT as network type. The guest OS does not have to know about VPN.
Depending on the type of VPN, there is an option under Advanced in the network system preferences, to send every traffic via this VPN connection.

As I said, it depends on your type of VPN, but in general, follow this:

1. Get your VPN connection running on Mac OS. If available, select "Send all traffic over VPN connection": System Preferences → Network → (select your VPN connection on the left side) → Advanced...

2. In VirtualBox, set Attach to.. for the network adapter to NAT.

As all traffic is now sent over your VPN, the traffic generated by VirtualBox is sent via your VPN. Of course you cannot see any servers or whatever inside your guest OS as it is behind the NAT but you should be able to connect to servers, etc. via IP or name.

It should be fine if you connect the Mac OS via VPN and configure VirtualBox to use NAT as network type. The guest OS does not have to know about VPN.
Depending on the type of VPN, there is an option under Advanced in the network system preferences, to send every traffic via this VPN connection.

It should be fine if you connect the Mac OS via VPN and configure VirtualBox to use NAT as network type. The guest OS does not have to know about VPN.
Depending on the type of VPN, there is an option under Advanced in the network system preferences, to send every traffic via this VPN connection.

Update:
As I said, it depends on your type of VPN, but in general, follow this:

1. Get your VPN connection running on Mac OS. If available, select "Send all traffic over VPN connection": System Preferences → Network → (select your VPN connection on the left side) → Advanced...
   Advanced Settings http://img682.imageshack.us/img682/3091/1003220001.png

2. In VirtualBox, set Attach to.. for the network adapter to NAT.

As all traffic is now sent over your VPN, the traffic generated by VirtualBox is sent via your VPN. Of course you cannot see any servers or whatever inside your guest OS as it is behind the NAT but you should be able to connect to servers, etc. via IP or name.