correct, there is no visa on arrival in Vietnam anymore. You, as a Brit, gets a 45 day visa exempt entry to Vietnam. Most countries need to have either a visa in their passport or the evisa approved before being allowed on the plane to Vietnam at the airport.
you apply on the official website and pay USD 25, or USD 50 if multiple entry 90 days, get approved, print out, give that to immigration officer and get stamped in. No further money required, or letters, unless you've gone through an agent and been fleeced.