Lower hull front should theoretically be vulnerable. Wouldn't the turret also be penetratable by the M8, though only at suicidally close range?
Hull sides, behind the tracks, under the sloped sections, should also be vulnerable.
I've killed a T-34 with an M8 before, but that was only when I was hiding inside of an undestroyed barn while he happened to park within my field of view and at close range. Tracked him, and he was either inable to get his gun on me, or didn't see my fire.
Closest comparison to going against a T-34 in an M8 is going against a Tiger II in a T-34/76 without HVAP. You can't realy hurt him from the front, you have to hit the right place on the sides, and without any deflection, going in behind doesn't help a whole lot aside from forcing his turret to traverse further, his gun can punch a hole through one side of your tank and out the other in escess of 2000m, and he's a bastage and a half to kill if the driver is even half smart.