Hey, you have to be rooted in order to flash a new rom. Just find ANY tutorial on rooting the Vibrant on XDA, they're all pretty much the same. Before flashing anything make sure you know what your stock firmware and kernel is so you can always go back to it in case you want to get official updates. You can flash different kinds of roms (including your stock rom) using odin. it's really easy to use. There is tutorials on odin on xda too, anyone with an average or higher IQ should be able to handle them.

 

well you can try setcpu now that you have overclocked your phone, but be careful with it if you don't know how to use it you can damage your hardware. do a nandroid backup using rom manager in case you screw up somewhere. you can do that by just rebooting in recovery and then backup. that's as far as it goes man.

 

Well the 1.1 does not include Voodoo unless you flashed the voodoo on top of it. SetCPU is only used for NON CORE Fusion versions. The newest Fusion does not need to use SETCPU

as far as going back to stock, which I dont know why anyone would want to, here is a link to accomplishing that. If you are switching back simply for Froyo, its really not that good. And once the custom ROM makes get the final release they are going to be cooking some great replacements.

Here is the link

AndroidSPIN [GUIDE] How to Restore or Recover Your Samsung Vibrant

This will bring you back to bone stock like out of the box, make sure to follow to the letter

 

Well did you install the Jacs Kernel over the top?