I'm going to try to make it a little easier for you to root cause I know sometimes Android can be a little overwhelming to beginners. It scared me the very first time I ever rooted a phone.
First and foremost, make sure you have something called Odin. No its not the Norse god, its a program on your computer to get it back to stock if you brick(render it useless) your phone. I've bricked my phone and its not that big of deal at all as long as you have the proper tools to work with.
Refer to this thread for all the files that you need to have on your comp before you get started:
[ROM] OFFICIAL T-Mo Vibrant JFD ODIN/Nandroid Backup Images 7/29/10 (Tested/Working) - xda-developers
This is video to give you a visual on how to do it:
http://www.youtube.com/watch?v=AnY7P14JSWQ
Keep in mind that you don't need odin unless you are flashing a custom rom or flashing it back to stock.
Now to the customizing stuff
This is a video on how you can root your phone the easy way:
http://www.youtube.com/watch?v=U508p2jRtlg
This is the site where you get the root zip file from along with detailed written instructions if you want to read it also:
http://theandroidsite.com/2010/07/18/how-to-root-your-samsung-vibrant-or-captivate/
Applying ROM's
Your phone should be rooted by now so watch this video on how to install a ROM on your phone:
http://www.youtube.com/watch?v=OCDsAHfoVwc
Keep in mind that you don't need OCLF in the first step that he states. I don't even think it has a root option anymore. It doesn't matter cause you rooted your phone already.
If you just visit the rooting section of this forum or go to xda, there is a plethora of roms you can choose from that can either be flashed via ClockworkMod on your phone or flashed through Odin. Its really just a preference.
Thats the best I could do to help you out short of doing it for you myself. If you can't do it now, well then the android platform really isn't for you.
Trust me, I've been rooted and custom rom'd since I've been introduced to android. It really is the best phone OS out there. The Vibrant is a badass phone but I wanted to throw my phone across the room b/c of how shitty Samsung is about broken promises and updates. Tmobile's customer service has gone down hill for reasons I couldn't tell you.
I still stick by word in saying that a person shouldn't have to go through all that to get a $500 phone to work like it should have from the get-go. But rooting and custom ROMs is an option and a really great option.