I found out it is too late to get a 3 week visa for Vietnam, as it involves leaving my passport with them for processing. I have opted for the express two week visa. This means I will have to cut my trip to Vietnam short and spend a little more time in Hong Kong.
I spent the day up in London and went to the Vietnam Consulate. You have to fill out a form (available online) and leave that and your passport with them for about an hour and they stick the visa into your passport. It cost about 55 pounds (UK).
Its a shame in this day and age that I still had to travel up to London to get this done, but I took a walk through Hyde Park and down to Westmister, so I got to see some of the great tourist sights that we sometimes forget we have in our own country.