You may install Vegas to 2 PCs and use it on 1 at a time. If you want to install it to a 3rd PC you will want to log into your account and deactivate one of the other two installs first, then install and activate Vegas to the new PC.
You may deactivate an install slot on the license by doing the following:
Go to www.vegascreativesoftware.com
Log into your MyAccount
Click MyProducts
Click "show more details" under the Vegas serial
Click "deactivate" for the PC you want to deactivate from