It just a solenoid valve.
As part of the control of evaporative emissions, the fuel tank is sealed and fumes are adsorbed by a charcoal canister, On your car this is behind the fuel tank.
When the ECU judges conditions are right, it opens the solenoid valve in the device you are asking about. Vacuum then transfers the vapours to the engine so they can be burnt.
A vacuum leak at the point you are talking about, and causing the symptoms you described, is a very common Focus problem; a mechanic familiar with the car would have diagnosed it in seconds. (The initial problem of the split in the radiator header rail is also known; usually you get a slow leak long before it fails completely. You do check fluid levels at each tankful of fuel, don't you?)
It won't do any harm to run around with the vacuum pipe plugged until parts are to hand, but don't forget it!
Chris