You're best bet would be to email Interplay. Other than that I think it'll be very hard to get the illustrations, since I can't think of a reason they'd be released to the public. Additionally, Interplay might still own the rights to the image, in which case it's extremely unlikely you'll be able to get a hold of it.
Out of curiosity, why do you need it?
edit: depending on how badly you want it, you could vectorize the raster image yourself, or pay someone to do it for you. If you opt for the latter, you might want to check out
Deviantart - if memory serves they had a subsection of their forums dedicated to hiring artists.