Assuming you have bought a serial key for the most recent version of Vegas you should make sure you are trying to install and activate that version. If that is not successful please repost your question to the forum linked in that post.
Yes, if you have any Vegas Pro issues, go to the Boris FX Help Page for activation issues.
Also, which version of Vegas Pro are you trying to activate? You can not buy a serial number. You would have had to purchase the software via Steam, Magix or Boris FX. Purchasing from them would have included emails with detailed download and installation instructions.