While they don't have one for the Vivid/Raider yet, I've always used ProClip mounts (Brodit outside of North America). They come in two pieces, one custom fit to the vehicle and one custom fit to the phone, and I like it very much. I've emailed them to see if they'll make a Vivid mount, but they gave the standard "We don't know what our plans are" answer. If people are interested, email them, maybe if they get enough questions, they'll actually make one!
Current Phone: HTC Vivid (White, Stock Rooted), Asus Transformer Prime (not yet rooted)
Past Phones: HTC Incredible S (Black, Stock Rooted), HTC Hero (White, VillainROM), HTC Dream (White, Too many different ROMS to list!)