Had the same problem. Thought it was just me :) Okay, uninstall VV3/4 or whatever. Then go to program files and delete the folders. If you also had the beta (like I did) make sure you delete those files under "Sonic Foundry Beta". In any case, delete all folders/files for VV. Then go to regedit and delete the VV keys in HKey Local Machine/Software/Sonic Foundry. There should be a Vegas key on the left and you should totally blow that away. Also in Software you will see a key for "MainConcept". Be sure to delete that too. Now you shouldn't have to reboot. Reinstall VV4 and there you go .. . Mine worked after that.
I did the same as pcfreakx and it worked until i shut down the app and restarted, than i was in the same boat (no custom button). The way i fixed it was to install DVD Architect, after that it works fine. Its a drag u need to buy DVD-A to make vv4 work. Hopefully it will be fixed in the first update.