The Vulcan supports multiple partitions, so 32MB isn't a hard limit. The problem is that the firmware only supports a limited amount of CHS combinations, similar to the old BIOS drive types before "User Defined" came about. I wouldn't be surprised if the table is close to or identical to them. The Vulcan ROM is completely inflexible and refuses to work if it detects something not matching its on board drive table.

http://webpages.charter.net/danrollins/techhelp/0054.HTM

http://vintage-pc.tripod.com/types.html

Here is the semi-official Vulcan drive list: http://mirrors.apple2.org.za/ground.icaen.uiowa.edu/MiscInfo/Drives/vulcan.specs