What about replacing the engine with an Isuzu diesel motor? That way its possible you could run on WVO or a Bio-D blend and hopefully make the saving in fuel costs essentially pay for the engine swap?

I'm sure the folks at www.biodieselnow.com would be more than willing to help.

Now I come to think of it (says the Scot who drives a diesel Yaris) has anyone ever made the swap from petrol for diesel power in their VX? Judging by the prices being fetched by VW TDis in North America (I'm a member of a coupe of other US-based forums) a diesel VX would be worth 10s of thousands of $...

Good luck with whatever you choose...

Alan