If you are referring to one of the Vegas Pro 18 versions (but nor Edit), that version did come with Sound Forge Pro but it was version 14, not 15.
If you need to get back to the forum about this issue, please provide information about which Vegas version you are referring to along with a background of any recent changes to your Vegas history, such as , "i had Vegas Pro xxxxx 18 but changed to Vegas Post 365 after Vegas Post 19 was released'. Also, what version of Sound Forge Pro are you referring to?