First, does the upgrade change any settings/preferences on the computer (Safari bookmarks bar, mouse sensitivity, folder organization, dock, etc.)? And does it delete any data (pictures, music, documents, softwares, etc.)?
No. Back up your system anyway as a precaution against the unexpected. Use Time Machine or "clone" software and ensure the backup is viable.
Second, my iMac is early 2013. It has 8 RAMs, 2.7 GHz Intel Core i5. Will upgrading to Mavericks damage the performance? Also, will I get all the advertised features?
Respectively: Mavericks will result in measurably increased performance, and yes you will be able to use all its features. Just make absolutely certain to eliminate any and all "anti-virus" or ill-conceived "cleanup" and similar junk from your system first. Most of those products will slow down your Mac, or worse, no matter what OS it runs.
Third, I also have an early 2010 15-inch Macbook Pro. It has 4 RAMs, 2.53 GHz Intel Core 2 Duo. Is the upgrade recommended? Or will it slow down the performance badly? The laptop is still on Snow Leopard.
Yes, though keep in mind that any old PowerPC apps will not run with any OS later than Snow Leopard. Mavericks will run fine on that model MacBook Pro and it is likely to result in increased performance due to enhancements in memory and other resource management. The same caveats apply regarding third party software, and back up your system first. Snow Leopard to any later OS is a quantum leap, but if you are already using Mountain Lion on your iMac you will have already become accustomed to it.
Upgrading OS X requires Spotlight to index hundreds of thousands of new files. That can take the better part of a day so initially you will wonder why your Mac is running so slowly and using so much battery power. That's normal, and your Mac will run faster when it finishes. If it doesn't, the problems causing it are most likely to be incompatible third party software, which is the reason for removing "anti-virus" and related junk and verifying Mavericks compatibility first. Only you can know what's on your Mac and Apple has no way of testing everyone else's software. Some developers have been lazy despite having many months to prepare for Mavericks.
Lastly, I have Windows Vista installed on the Macbook (Bootcamp). Will I still be able to use Windows Vista after upgrading to Mavericks? I know only Windows 7 or later can be installed on Mountain Lion. So I was wondering whether upgrading to Mavericks will still allow me to use Vista.
Boot Camp obviates what is installed on Mountain Lion. Whatever you have installed on a separate partition will be unaffected by upgrading to Mavericks.
