The airport elevation does not effect the Vr as an Indicated Airspeed although the True Airspeed will be higher, hence longer take off roll etc.
I am not too struck on the idea of "Vr" in a light aircraft - it's more a question of accelerating to the correct speed and then selecting the take off attitude and then waiting for the machine to fly itself off the ground. Don't expect to immediately get airborne when you select the take off attitude!
Also where have you set the trim for take off - might be worth experimenting a little with this.
However, my best suggestion is to find a good instructor who is experienced on the type and take some training from him/her.
Hope this helps