Okay guys, there's a thread here about how the Galaxy S II Epic 4G Touch battery might fit the Vibrant with no need for a new back cover. The rumor is true. I have two of these batteries, and have verified that mine are legit, and they fit perfectly. My phone actually has never lasted so long on a single charge since I switched to CM7.1, and I won't be going back to any battery of less capacity for as long as I own my Vibrant!
Here is the official page for the battery from Samsung
Here is the website I purchased from.
NOTE: I have removed the note about my batteries being fakes because after some research I have determined that Samsung actually has no idea what a serial number is, and is actually putting batch numbers on their batteries, so having the same number isn't a bad sign.
Here is the official page for the Extra Battery plus Charger from Samsung
Here is a website that seems to have the battery charger combo in stock and is selling it for $10 cheaper than Samsung MSRP. I DON'T vouch for them yet, because I haven't purchased from them ever.