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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: http://www.foodoo.sunreal.com.au/info/date_info.html