In short you install Clockwork Recovery, Download a ROM and move it to your phones internal memory, and flash the ROM in Clockwork.
Head over to XDA-Developers forum, go to the Software Development section of the Vibrant forum. There you will find a hand full of custom 2.2 ROMs from different developers. Which one you choose is totally your preference, read through the descriptions of each ROM and choose. Feel free to try as many ROMs as youd like. More detailed instructions on flashing ROMs can be found in that forum also.