With PC based VW, all versions inc 8-10.5, I have the same problem. My plots of all sizes are given a huge 1" plus margin at the right and top. I have worked around by resetting the plot size to much less than 24 x 36 (22 x 34). These wind up being centered but have big margins of course.
However, 1/2 size plots dont work as they are always pushed off the page by those huge margins. I have looked into the printer driver and VW to fix-no luck. VW techs tell me to talk to HP and HP says talk to VW.
Anybody know how to deal with this?