Vanilla just refers to the version of Honeycomb, or any other version of Android, that has no custom UI built over top of it. The major benefit of vanilla android is it is easiest to update and will be the first to get updates as there is no extras involved with manufacturers custom UI's. When you have a custom UI like HTC's Sense or Motorola's Motoblur the updated OS has to be sent to the specific manufacturer so they can code it with their additions. Hence the reason for the longer waits.