This article at Wired suggests the best way to deliberately trigger a volcanic eruption. The writer, a professor of geoscience, outlines the way this would be done, but runs into a snag at the end. His problem is that once he's done what he needs to do to start the eruption in motion, he can't guarantee when the eruption will occur. It might happen right away, or the volcano might take as long as a month or even a year to get its Vesuvius on.
"Sheesh! Amateurs!" Ernst Blofeld was heard to say.