I think you need to spend more time thinking that through. Let's say you get 17 MPG now and diesel boosts you to 34 MPG at a cost of $10K. Assuming both gas and diesel cost around $3/gal (a stretch, since diesel is often more expensive than gas), you'll need to drive well over 100K miles on the new engine just to break even. And that's assuming you have the $10K in cash (which you may very well have after your deployments) - if you have to credit-card the replacement cost for several months/years it'll take even longer.
Well, maverick, now that gas prices are over $4/gal and are supposed to be going over $5 within a year, feel free to recalculate the mileage cost, if you will... Was the swap ever done? What happened with costa's ideas?
Around 70K miles at $5/gal or 87K miles at $4/gal. As I implied before you need to be in this swap to help the environment rather than your wallet, unless of course you plan to daily drive your VX for many, many years to come.