I don't have much time to go into detail, but I'll try to provide something of an overview.
Rooting is merely a process by which you gain greater access to the phone's OS. It will not make the phone faster and you will not notice any difference in the phone's operation by just rooting. The advantage to rooting is that it allows certain apps to do things they would not otherwise be able to do. The Vibrant is a very easy phone to root.
The Vibrant is somewhat infamous for having lag. Much of it has to do with the proprietary file system used by Samsung. I noticed this condition when my Vibrant was new, even before I ever heard of a "lag fix." Some say their Vibrants have no lag; most agree that there is enough lag to be an annoyance at one time or another.
People have developed workarounds for the lag; these involved manipulation of the phone's file system and overall are safe to use. The most popular lagfix is the "OCLF," available for free on the Market. There are more sophisticated lagfixes, most notably Voodoo, but until you are well-acquainted with your phone and have done a LOT of reading I strongly suggest you steer clear of it.
Learn about
Odin. It can save your a** if you get into trouble and brick your phone.
Here is a good resource for you:
[REF]***Vibrant Directory(Bible)Rev2.2--Time: the 4th dimension--***(11/16/2010)*** - xda-developers
As for the OCLF, go here:
RyanZA's OCLF 2.0 - Android app on AppBrain
Ask questions before doing anything about which you are uncertain. Better to ask and possibly get flamed a bit than to brick your phone when it could be easily avoided.