I always draw into my mouth first (but I did that when I smoked, as well). I guess I draw on it for maybe 6 seconds or so, then inhale. I still get a great TH with 12mg nic (usually, but I do have one 18mg) and all VG juice.
It seems the longer you inhale you will produce less vapor.
Statement: 3.14159265358979323846264338327950288419716939937510 seconds, on average (Pi, for you non-mathy types).
Observation: I tend to like taking a longer slower drag with some flavors, shorter faster drag with others. It's all subjective.
Statement: I typically take a short drag, inhale immediately, exhale through the nose while taking another pull. On some flavors that can cause the taste of the flavor to increase dramatically for either the better or the worse (menthols come to mind more for rapid-gaining flavors).