The phone will be ROOTED after you flash to the new ROM. It is included in the ROM already. You just need to flash it.
You will flash your recovery menu then your ROM.
TeckOne you first run romtools to flash your recovery program on your phone, that's when you press "Home-volume down-power" but FIRST you need to look at the ROMtools folder on your computer, check that there is an image file called "recovery" it should be "vrecovery" if you have an Optimus V phone. Make sure to rename it to "vrecovery". (do not connect the phone to computer yet) After that, run ROMtools and on the cmd menu screen run option 2 "Flash recovery to phone" then option 1 if your phone is a "V". After that press option 1 and install the drivers. There will be 3 so just wait until its done. After the drivers install connect the phone to the computer until it is recognized, then the ROOT program "Superoneclick" will start, press SHELL ROOT then wait for confirmation. After it finished and you keep pressing any key it will reboot the phone into the NEW recovery screen that will allow you to flash ROM/recover/backup and many other things from and to your SD card.
(Just remember about the image file in the ROMtools folder and make sure it says "vrecovery" I guess it's if you have a Optimus V, donno all I know is that it would not work for me if I didn't.)
Hope that helps,