That happened to me also, and it was my very first flash on the Vibrant. It soft-bricked, but I just kept at it... got back into download mode... and tried again. It worked. As far as I can gather, as long as you can still get to download mode, you are golden, even if it takes a try or two. It seems that Odin is sometimes temperamental. It only takes a couple of minutes to flash via Odin, but when you are sitting there waiting for the PASS notification, it does seem like an eternity the first time you do it.
I got a FAIL the first two times on the very first attempt. It's a bit unnerving when you aren't sure it'll come back.