Ello : ) pratik,
After you install your Redhat in your 8GB of unpartitioned space,yes you will be able to boot from Vista and windows also ..
If you follow the normal installation your linux would be the default loader and you will have to press h for you to show the other boots.
I did a normal installation on my PC n am pressing h everytime i start my computer if i want to go to windows.
but there customized options on, what is the default loader and how much time it has to wait n stuff, something to do with GRUB i suppose. i ll also like a answer for your question.