Vestapol refers to the chord voicing, not necessarily the Key. The most common chord played in Vestapol tuning is D Major, so your tuning is: D-A-D-F#-A-D or E Major, so your tuning is E-B-E-G#-B-E.

But it is the relationship between the open strings (or the voicing of the chord) that is described rather then Key that the term defines.

I don't know about the origins of the word. Here are two webpages with more info:

ONE

TWO