If you are using Vegas Pro 14 or later, go to the Vegas Pro lisitng in My Products in your MAGIX My Account. There, you can de-activate one or both current activations - this can only be used once per month.
Older versions of Vegas Pro than 14.0 are licensed under a system (as previously used by Sony) which permits use on multiple computers by one user; after an unspecified number of installations on different or updated PCs you may need to contact support to reset your "installation count".
More information about de-activations can be found at point 9.