If warped front rotors are replaced and the existing brake pads still have 70% left on them, is it advisable to also replace the pads? The uneven pads can cause a vibration when braking - will this increase the chance of warping the new rotors?
Yes to both questions.
You can reuse the pads, but you must knock down any glaze on the pads before reinstalling them.
Take a sheet of 800 grit sandpaper and lay it on a sheet of glass or on another flat surface. Now take the pads and rub the face of the pads on the sandpaper until the surface is dull. Install the pads, and follow the proper break-in procedure to seat the pads to the rotors when first driving the vehicle.
The pads should show equal wear. If the friction pads are tapered, or badly grooved, they should be replaced. To lessen the chance of the new rotors from warping, tighten the wheel lug nuts by hand, in a crisscrossing pattern and to an equal amount of torque. After heavy braking, or stop and go braking, make your stops slow rolling stops for the last few feet (creep to a stop). This will help to keep one spot of the rotor from keeping a higher heat, from braking friction, and warping. Uneven heating, and cooling, of the rotor, can cause warping.
Thanks for the replies.