What is the exact procedure to install Zorin with dual boot on Windows Vista? Any windows prep beforehand?
Check the downloaded iso hash! It proves its not corrupt. Know which version you downloaded and need (i don't recall a 64bit Vista, but i could be wrong. It must be the same as your windows install. Just like i believe mbr was still being used, but it was close to the change, so could be uefi).
Backup anything important!
Make sure, in windows, you check power options for, and turn off, fast boot and hibernation. Fast boot may only be windows ten, but I'm putting it here in case. It will be listed in the what the power button does link or buried in advanced settings.
Turn off secure boot and fast boot in the bios.
Use gparted in live image to resize the windows partition and boot into it before installing. Take note if there is an efi partition. Make sure check disk and other disk tools aren't scheduled. All of these things will lock the drive and make the install impossible or problematic.
These suggestions are best performed in the something else option when installing zorin:
If there is an efi partition, you should have no problem installing using the uefi method (create root partition and making it bootable for root), otherwise you may want to create your root partition, naming it and marking primary, to ensure you don't have 4 primary partitions to start (recovery would also require a primary as well as any other bootable image on your system). Root should be 30 to 60gb depending on your application requirements and disk size. Must be mounted on /.
Depending on if you want a sharable data partition, create your /home partition larger than root (smaller than the drive capacity though for the sharable partition) . Configuration, customization and application files will be stored here, as well as anything you make or download. Ext4 is the default for root and home so i would stick with those file systems. I recommend you mount it on /home.
The third partition is optional. It can be called anything you wish, mounted anywhere (usually on /mnt) but must be formatted ntfs and marked extended (mbr requirement). This allows both OS's to see and access it. Don't create it in windows. This ensures it's not claimed by windows for something and made unwritable to other OS's. Disk check and other disk utilities may do the same while in windows. Windows is greedy.
After all this you should have a simple, easy to manage install. I'm not saying you won't have issues, but the install should go without problems.
I could give you things to read that would prepare you for some of the possibilities, but it's better to deal with the known than try to prepare for every aspect of unknown, which you will probably miss something if you did attempt this. Let us know which version you install and how it goes (even if it is without issue).
Fast Boot began with Windows 7, I believe.