If the vibrations are only present during braking, and it suddenly manifested itself during one or two brisk stops, then I would look into disc glazing. From the sounds of it, your car had been gently driven, and when brakes dont get hot enough they tend to accumulate brake residues.
Perhaps a few hard stops got your brakes hot enough to begin releasing some of the residue, but left some uneven deposits.
I would personally go find a deserted wide open road and complete a brake bed in procedure. Google or search the site. It is not a difficult process, but will require good driver judgement because higher speeds and hard accel/braking will be involved.
But then, I could be something completely different.
it can be pretty hard to troubleshoot over the Internet!