We had the same vibration from the front under braking occur, and the Lexus dealership turned the rotors under warranty - that was at about 20K miles. (we live on a pretty steep hill, and it's tough to get the rotors warmed up before you need to really brake hard).
We went back for a 30K service and felt that an alignment was due (due to a curb/parking incident), and the same dealer told us that all of the rotors needed replacement, as they were not within tolerance (not enough material to turn). The estimate was $1000 for all 4 wheels. Brought the vehicle home, pulled the front wheels, mic'd them (still plenty of material for a mild turn/cleanup and stay within spec), took them to the local auto parts store, had them turned for $10/each, put them back on, and they worked perfectly for another 25K miles.
I've been told the one of the fastest ways to get a warped rotor is to apply heavy braking when the rotors are cold. They get hot-spots on them, or don't heat evenly, and warp. If they have a chance to warm up, (light braking will do it quickly), there may be less chance to warp them.