What does "vanilla" mean when referring to AppleScript?

The term “vanilla” is used when referring to AppleScript, as installed with the Mac OS. In other words, “vanilla AppleScript” involves no applications or third-party scripting additions.

This term has been used in various contexts, and its common jargon meaning is “standard” or “ordinary” (that is, with no add-ons). Check this for more info.

In AppleScript we have also the “French Vanilla”, which is a very old method writen by Emmanuel (author of Smile) used to convert months to numbers (eg, March => 3). It is very fast, but now obsolete as for AppleScript 1.9.2, which provides this coercion (eg, “March as number” → 3).

Richard Morton explains it all: RooDooLounge:Info:DateStringInfo: