Although the wormwood added to these drinks is probably for tradition, the alcohol content of vodka is not strong enough to get the necessary oils from the wormwood that made Absinthe so popular.
The ingredient from wormwood was thujone which acts upon the same receptors in the brain as THC. The un-official guide to drinks does have the commercial recipe for the distillation of absinthe.
Also check out the book Absinthe: History in a Bottle.
Source: Kevin Blankenship < Blankkd@wku.edu > - 1997-08-27
Warning: Wormwood can be habit-forming, cause mind-altering changes and may lead to psychosis.