No idea on the difference offline versus online, but agree it's probably lag related.
As far as attack method, I tried once diving in at near vertical, and wound up as a large smoking hole right next to the tank I tried to attack.

A lot easier and reliable method is as previously described, between 30-45 degrees decent angle, my method is to get the bird as slow as possible to give me the most time to get the shot lined up.. I am just about completely throttle off, 1 notch of flaps, pretty much gliding in, just enough throttle to keep on glide-path and not sink too much. I have my convergence set to 200, and don't fire until 200... ammo is too precious.
Shot placement is critical, unless it's a Panzer, the AP rounds seem to give me a kill on any panzer no matter where I hit them.
M4's, T-34's, and Tiger I's are a bit tougher, and require a hit right in the turret or engine cover to disable them. Frontal hits are pointless almost, and side hits to the main body of the tank seem to be hit or miss (pun intended

) and only seem to do damage if but rounds hit at once in the same point perfectly at convergence.
Tiger II's I've only ever killed once, and it took every round I had once I had tracked it. Someone calls out "King Tiger!" I get the JU-87D and strap on the VW......