the book says the fuel tank capacity of the mk5 is 45litres. I assume that's from a 'safe' full level to the reserve (which I believe to be about 7 litres).
So in theory if you run it dry, you can get 52 Litres in, but that's assuming you're running on vapours.
I'd recommend against the venting trick -you're probably okay this time of year, but in the summer you run the risk of bowing fuel out of the breather, as others have mentioned.
If you're that worried about carrying enough fuel, buy a jerry-can.