All right, the way I rooted my Vivid. No issues whatsoever.
STARTING FROM SCRATCH
1) Gather all these files:
HTC Drivers:
http://db.tt/Pi0WLxUf
Fastboot Folder:
http://db.tt/2Zg3UeCw
SuperUser Zip File:
http://db.tt/n4tr8lwi
Recovery:
http://db.tt/oJNL1hPs
(WildChild, if you see this and wish for me to take down the link to your custom recovery, please PM me. And to others, please alert him, I don't want to be in trouble is all.)
2) Install the HTC drivers, place the Fastboot folder on your desktop for easeness.
3) Backup your apps, contacts, text messages, and anything else you want to keep, because later, you're gonna have to unlock your bootloader (shown later).
4) Unlock your bootloader. Now, in order to flash a custom recovery, backup your ROM, etc., you need to have an unlocked bootloader. You can do this at:
Welcome to HTCdev and select Unlock Bootloader. You need to register, so just make an account real quick and you're good to go. Select the HTC Vivid and follow the instructions. NOTE: UNLOCKING THE BOOTLOADER WILL FACTORY RESET YOUR DEVICE, THAT'S WHY I RECOMMENDED BACKING UP WHAT YOU PLANNED TO KEEP.
5) Place the SuperUser zip file on your SD card.
ALREADY UNLOCKED THE BOOTLOADER
All right, you made it this far, why stop now? Time to flash that custom recovery and root your phone! You can mod it later, I'll explain later. All right, so now, place your phone into Fastboot mode by holding the Volume Down and Power button until you see the HBOOT screen. From there, select FASTBOOT, and connect your device to your PC. Confirm it says FASTBOOT USB in the red banner. Let's do this!
1) Place the downloaded recovery [wcxrecovery.img] inside of the Fastboot command folder.
2) Open a command prompt in that directory and type the following command exactly as I type it.
fastboot flash recovery "wcxrecovery.img"
And then hit Enter. You should notice a bar in the top right of the screen going up. That's normal. Wait for the command prompt to tell you that its finished, then disconnect your device from the computer.
3) Exit out of the Fastboot menu and navigate to the RECOVERY and press the Power button to enter. You'll now see options to flash, wipe, etc. Using the Volume buttons, go to "flash zip menu", locate where you placed the zip file, press Power to flash it, and voila! Your phone is rooted after it says complete! Just reboot your phone and enjoy root access! Thank me if I helped!
Here is what the recovery will look like:
Sent from my HTC Vivid WCX using Android Central Forums