You can dual-boot quite happily: Just check the relevant Options as it Installs.
BUT, you must recover/re-install/fix Vista first to your satisfaction first, coz Windows is unforgiving and will wipe Mint if it's there already!
The Mint Installer will detect any existing OS(s), ask you which you want to keep and then offer you a decent size Mint Partition. You can resize the latter at that point if you think it not enough, but I find the default options are always good. 50GB should be far more than ample for your usage. (Assuming your Hard Drive is 120GB or more).
It really is very easy - I managed it!
PS: Having dual-booted a Linux Distro of one sort or another with Win7 for a couple of years, I recently "let go" and did a clean install of Mint only. It was very liberating! No regrets.