ISTBC, but I think the online system works as follows:
Booking "Upper Class (lowest)" will give you the cheapest fare available at the time (whether that's Z, D or J) and booking "Upper Class (flexible)" will always give you a J fare.
The problem with the online system is that, unless you go almost to the end of a purchase, you can't find out what the ticket class is (it's only displayed on the final confirmation screen before you actually buy the ticket). So, until VS make the online booking engine the way we'd all like to see it (with an option to select a specific fare class), the only way to truly make sure you get a D fare is to ring VS and ask directly for one.
Given that there's no incentive for booking online at the moment, you're not disadvantaged by this

Alternatively, you could try Expedia & Travelocity - IIRC they display fare classes once you've selected your flights...
Cheers
Michael