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Variable valve timing is automatic and, generally speaking, improves the efficiency and/or power generation over a broader RPM and load range. It's not something for a driver to take advantage of.
I don't have experience with that particular engine, but generally speaking, the keys to improving economy are conserving momentum and keeping your speed down.
Diesel has two main advantages over gasoline engines to keep in mind:
1) Gas engines have reduced efficiency at part throttle, while many diesels do not.
2) Diesel fuel has something like 25% more energy in it than gasoline, but is also more expensive in many parts of the world, so you can't really compare miles per gallon between the two.
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