Vista is taking up space on your computer and you want to get rid of it. That's understandable, but in a few cases you might want to have theOS in your computer anyway.
Please allow me to share my practical experience. My computer is a multiboot computer and I have finally decided not to bother removing systems I don't use. Nowadays hard drives are cheap, at least the common IDE drives. I normally have two drives installed in my system and I believe the poor performance is a little bit better than when I had just one drive. My power supply has more than enough power to supply my hard drives and my graphics card. So I see no point in removing one of the drives. As for drive space, Windows Vista is a hog, but it can reduce down to about 10 GB. One of my drives is a 250 GB drive, so I did not bother to remove the stuff from the system.
My reasoning for this is because in times past I have tried to move my operating systems from one place to another and repair the system and it was just a big hassle and then I would have some doubt as to what about the system would be reliable after doing so many changes and mutations to it.
Now for the theoretical problem. It may be that many users don't understand that once you install an operating system in a certain configuration it can be very hard to change it. With regard to how the system boots up.
If you wish to have just Windows 7 as the operating system, you would need to do a repair of Windows 7 so that it will boot by itself and not depend on the other operating system to do this start up. As for myself, I have done that and I don't like it. I prefer to do a fresh install rather than try to do a repair. But in my case, it is my practice to have backups of everything and that makes it easier to do a clean install been rather than trying to repair a defective install.
Anyway, I just wanted to share that with you. Of course it really is possible just to repair the window seven installation and it is quite a learning experience and it may work out well for you. But just in case, be sure you have some kind of backup of any thing that is important to you.
Please excuse spelling and grammar errors, a I have limited ability to proofread my own stuff. This is from speech dictation.