Pantech Crossover Root

[Yami]

I have a GS2, it's pretty much the easiest-to-root phone in existence (go to XDA-Developers, find your version, search 'cf-root' in that forum, follow instructions).

 

[Puddle Jumper]

I think the Vivid has some level of locked boot loader so you may not be able to root it until a workaround is developed. The regular Galaxy S2 is a piece of cake to mod as Yami said. Just be sure to use roms for the AT&T version as ones intended for the international model do not work properly without some modifications.