when you get the phone dial *#7465256# and if it all says off then your phones already unlocked so ignore the rest of this.
otherwise heres a step by step as best as I can figure for you:
1. Use Odin to get back to stock JFD or use the flashing method:
make sure there is no lagfix enabled!! (you might ask the guy whos selling it if he had one, or search the sd card for a lagfix folder or something)
1.1 Odin Method
Download these files and follow Justanothercrowd's instructions to install and use Odin by going here
(these files will get you back to JFD 2.1 stock unrooted
1.2 Flashing Method (I've never done this myself so I'll let someone else post instructions if you dont want to use Odin)
2. Follow these
instructions to unlock
(specificaly by helroz, if you need to, read ahead and see what problems other people have had though)
3. dial *#7465256# in the dialer (as soon as you hit the last # it should work)
make sure everything says off (this means your unlocked)
4. Reroot your phone using one of two methods:
4.1 the guide found here
4.2 Or download the galaxy s rooter app or OCLF app from the market and follow the instructions in the app.
5. download the rom of your choice (If you would rather stay with the stock one then ignore the rest)
6. without unzipping or doing anything to the rom file, place it on the root of your internal sd card (the big one. it should be 13 GB or so)
7. download and install rom manager from the market.
8. open rom manager and choose the first option (flash clockworkmod recovery)
choos the samsung vibrant as your phone and wait, it will probably take a few minutes.
9. choose the second option in Rom manager (reboot into clockworkmod recovery)
the first time you do this you'll boot back into stock recovery, just hit reinstall packages and it will reboot into clockworkmod.
10. choos install zip from sd card.
11. choose the rom file you downloaded and wait while it installs
12. choose the option to reboot the device.
13 enjoy your unlocked rooted phone running your choice of roms.
EDIT: Make sure you have ADB (Android Debug Bridge) installed on your computer before running the script.
Also make sure that you have USB debugging checked by hitting (from the home screen) Menu >> Settings >> Applications >> Development >> USB Debugging
You can download and install the Android SDK (Software Development Kit) by following the instructions here
This will also install ADB.
another good idea is to make a backup of your efs file (I beleive this is in the instructions for running the script, but I thought I should say it again)
just in case something goes wrong.
this file is located in: the root of your data partition if I remember correctly, it should be labeled "efs"
you can use a root file manager (I use root explorer but it is a paid app) or you can use ADB to pull it.
you can also try this method by going here
and following the directions. ( I have not tested this method myself)
EDIT 2: you may want to download the drivers for your phone as well, if you haven't already that is.
you can get them here