Drive Train Loss is just that power lost due to your transmission, differentials, transfer cases, auxiliary gearboxes, multipliers, what ever it might be in the end.

Vapor Horsepower: is horsepower that magically disappears when you test the engine out... This is when you find out that your 285hp engine was built by a bunch of stoners on a Friday with big bag of weed and you only get 220hp... 65hp just vanished for no good reason other than the manufacturing process.