You can run the RUU as long as you have HTC Drivers (In this case, you have HTC Sync), ability to turn on phone, and a good USB cord.
1) Boot into HBOOT menu.
[Achieve this by holding Volume Down + Power until you see 3 Androids on skateboards on the bottom of the screen.]
2) Using the Volume Buttons, navigate to FASTBOOT and use the Power button to select it.
3) Connect your phone to your PC, and confirm your phone is connected by looking at the Vivid's screen and locate the red banner, it should say "FASTBOOT USB" and not just "FASTBOOT".
4) Run the RUU.
[If you don't already have it, download it here:
http://dl3.htc.com/application/htc_vivid_att_RUU_3.26.502.56_US.exe ]
5) Follow on-screen instructions.
6) Done.
Sent from my HTC Vivid WCX using Android Central Forums