I have several of the Westfalia Vanagon swivel seat mechanisms for sale. 
These are what is needed to convert a regular Vanagon front seat into a
swivel seat.  It comprises of 3 components and it is a permanent
modification as the base plate gets welded on top of the existing track
that your seat is attached to.

The price is $75 for each seat set.  They are in San Francisco area but can
be shipped anywhere.

Sorin

(650) 400-7810