1. A "vanilla" kernel is the official kernel delivered by the kernel development team, without any patches added by the distribution.
2. Many patches. Most of the patches are stable changes that the distribution has pulled from newer kernels. This allows newer functionality, with minimal impact on stability.
3. The vanilla kernel is only available as source, not rpm, and is independant of distribution (though some requisites may have to be resolved). It can be downloaded from here
. While it is not intended for use by inexperienced users, it is a great way to learn the kernel development process. A good resource for new kernel users is here