It doesn't matter if the wing is symmetrical or non-symmetrical ("cambered"). To perform a vertical roll, you need to put the airplane on the zero lift line. This is the vertical position where the angle of attack is zero. No AOA, no lift.
Even with a non-symmetrical wing like the one on an RV, there are still two intersecting lines which define this angle of attack thingie. One is the relative wind, and the other is the chord line. The camber doesn't mean you can't get to zero AOA. By definition, the chord line is still a line, and the angle between it and the relative wind can be reduced to zero no matter what kind of airfoil you've got.
As seen from the ground, these vertical rolls may appear slightly barreled because of the relationship between the wing and the fuselage, but that's a minor distinction which would only be of significance if you were in formal competition.
In theory no rudder should be required during this vertical roll since to zero lift line means aileron drag is the same on both sides. But this assumes your ailerons deflect equally.
The easiest way to learn vertical rolls is to first learn to establish good vertical lines. Consistently. Without them, you won't progress. Second, just try a quarter roll on an upline, and don't use full aileron deflection. You want to see what's happening. You want to be able to visually spot the point where the aircraft comes off axis. If you roll left, you'll have a slightly easier time of it since torque is going to be wanting to roll the plane in that direction anyway.
Since you mentioned hammerheads, I would master those before attempting vertical rolls. It's an easier maneuver. Vertical rolls are deceptively difficult. A reference for that zero lift line is also important, whether it be a piece of tape on the canopy or a formal sighting device on the wing. Here's a video of a vertical roll (a torque roll, actually, but the airplane is placed on that zero lift line and that's what I wanted to illustrate):
http://www.youtube.com/watch?v=7AplVT0b768
Also, a side note regarding the hammerhead: when you kick in that rudder at the top of a hammerhead (assuming your going to yaw left), the gyroscopic force from the propeller turning at max RPM will want to push you over on your back. The natural tendency is to push forward to on the stick to keep that from happening. Since you're slow, you might need a fair amount of that forward stick. Think about where the controls are now positioned: full left rudder, close to full forward elevator, full power, and by definition uncoordinated. In other words, the proper position for an inverted flat spin.
If you haven't had advanced spin training covering all eight spin modes (nine if you want to include knife edge spins, but I'll exclude that one since it's extremely unlikely unless you wanted to get into one), I highly recommend it before you start getting into this stuff. More than one person has ended up in such a spin, botched the recovery, and been killed. The answer to this is training, training, training.
RVs are great aerobatic airplanes, but they are high performance and have to be treated that way if you want to protect the aircraft and yourself. Especially for the less experienced aerobatic pilot, they're easier to overstress than many other aerobatic rides.
