A "vapor trail" and a "wake" are 2 different things. A vapor trail is the same as a con trail left behind a jet. It is more common in smaller calibers like .17 Rem. and hi-velocity rounds (4000 fps.)The vapor left in the air looks just like smoke and acts just like smoke, it drifts and can be spotted from almost any angle.
The "wake" however, is harder to spot. The wake is the air separating around the bullet just like a wake from a speed boat. The spotter needs to be looking right over the bore, behind the shooter. You don't need alot of magnification, but if you blink, you'll miss it.