open vpn in flash drive

i had configured openvpn in ubuntu and its running perfectly.who so ever is having client certificates is able to  access the network through vpn. now i want open vpn in flash drive as i have seen somewhere where two locations are connected through vpn and vpn is installed in flash drive. below is the description what i want.

1- vpn is installed in a flash drive and OS is Linux n that flash drive is than attached to a small hardware where v attach that flash drive v configure open vpn according to the need (means how many client certificates)

2- in that flash drive whole linux OS is not installed plz guide me what minimum should be installed in that flash drive so that vpn can be made.

                                       
                                              VPN hardware Description


-  flash drive attached to vpn hardware decription is as below

-  Vpn Box is having two ethernet connection,

-  eth0 is directly connected to adsl link
-  eth1 goes to switch.

now gateway for the lan is eth1

- it is having a small processor also
- 1 gb ram

may be i am not able to make u understand u properly, but this i have seen at client place. both locations are connected through small vpn box both vpn box are deployed at gateway level of lan as they both having ethernet card. so plz guide me.