I am not familiar with this option, but go ahead and click "Make this system partition". Before you do this, however, make sure that you have a Repair Disc handy. System Repair Disc - Create
Then you can delete the Vista partition. There is a good chance that you will no longer be able to boot Windows. At that point, boot from the repair disc you just made and run a Startup Repair 3 times or until it works. If it still doesn't work, make sure the partition is active.
Boot into the install/repair disc and wait for it to load
Choose “Repair my computer”
Select “Command Prompt”
Type “diskpart” and press enter. Do not type the quotes.
Enter “list disk” and note the number of the disk you want to install on.
Enter “select disk n” n is the number of the disk.
Enter “list partition” and note the number of the partition you want to install on.
Enter “select partition n” n is the number of the partition.
Then you can run Startup Repair again.