Welcome to Linux and Mint. I have a few suggestions for you before you start installing on your main system.
1) I am assuming that Vista is using your entire disk currently. That means you will need to shrink your Vista install to make some room for Mint. I know many will tell you you don't need to defrag your Windows install first, but I am not of that school. You would be well advised to take that little bit of extra time and defrag before you start this process. It just gives you a little more margin for safety when you shrink the Vista install.
2) I consider a Windows drive full at 80%. Windows doesn't like a full drive. It gets very slow very fast. Do a rough calculation of what the total size will be after you add any data/program space you see in your Vista future, then add 20% to that total. Don't shrink your Vista install below that size. If you can't get at least 10 Gig. for your Mint install, I would seriously consider purchasing another hard drive to add to your system before you attempt a Mint install.
3) Use Gparted, either by downloading the live cd iso or using the Gparted program on the Mint 4 live cd install disk to resize and partition your hard drive before the install. Depending on the space you have available and the needs of the next section, you may want to create an extra partition to hold your Windows backup image.
4) If you are concerned about the number of reinstalls Vista will allow, use one of the cloning or imaging programs that are available so if a reinstall is needed for some reason you won't have to use the Windows install disk. Ask in the forum for advice on which program to use. There are others much more qualified to help you with that chore.
5) There may be one or two more small partitions on your hard drive with the Windows install now. Make sure that you leave that partition arrangement the way it is. When you shrink your Vista install move the other partition back up adjacent to Windows where it was, putting the space you liberated at the end of the partition table. The unallocated space should wind up on the right most part of the graphic in gparted.
6) Read... There is a lot of information in the forum and on the Mint wiki that will give you a head start so you won't be going into anything completely blind. The search function in the forum is not the best I have seen but it is quite adequate for finding information on key words. Don't hesitate to use it.
7) A bit of philosophy to end with. You can go to two different extremes. You can insert the live cd, boot it up, push the install button and hope for the best. Or, you can study and learn as much as possible over the next two or three years, and then install. There is a happy medium in there somewhere that balances the risk of problems with the odds of a successful install. The point between thess two extrems is a personal decision. So do as much prep work as it takes to make you comfortable, but in the end you are going to have to actually install it for a successful conclusion to be possible.