That's a drag. You absolutely belonged in the priority immigration lane as a business class passenger on Vietnam Airlines. In the 3rd bullet point of "On-Ground Services" on the airline's own website is the published policy.

For the sake of clarity, I feel the need to point out that "fast track" is a paid service offered by various agencies giving the buyer access to the "priority" or "expedited" (in the airline's parlance) immigration/security lanes. Therefore, there's actually no such thing as a "fast track lane" and I doubt these words mean anything to the average airport employee, especially the non English-speaking security staff patrolling the entrances. Don't mean to be pedantic, but mentioning this in the hopes it makes a difference for passengers who qualify for (free) priority immigration at SGN. If even saying "Business" -- and showing the boarding pass that it's on Vietnam Airlines metal -- doesn't register with a particular thick employee, I wouldn't hesitate to get VNA ground staff involved.