The HP Z drivers have been notorious on their ability to fall back to the default settings. So it is not something new. First thing to do is make the default settings close to your most common used settings. Second: the printing shortcut settings stick better than temporary made changes so create some for the other choices you often make. Then the last if you need a special size use a name for a custom size that already exists and just change the size and save that size with the old name. The size will stick then. If you create a new size + a new name and save it you can be 99% sure the driver falls back to the default size, in your case 'letter'. Only if you go back in the driver and select that new name again it will stick. That has been my experience and not just in Vista. Use a size name like VarySizeZ3200 that does not indicate a special size and shows the temporary character otherwise you may get another issue when the application can recall printer driver data for a repeat job like Qimage can.
I use the PS driver sporadically so can not comment on the other question.
met vriendelijke groeten, Ernst