Volume in Vegas Pro can be set by a number of ways including: 1/ the volume level on the individual audio event; 2/ audio envelopes (aka rubber banding via keyframing); 3/ using the native Volume FX plugin on the audio event; and 4/ using the Mixing Console via View/Window.
It would be helpful if you could upload a screenshot of your Vegas Pro 15 highlighting the decibel level indicator that you see at the bottom left.
Event gain, track volume fader, track volume automation (envelope), sub-masters (if created and assigned tracks), last but no least, the master bus. There are many meter types, range and scale options as well. Plug-ins can also be applied at many levels which will affect and display volume, not to mention the pre and post fader options.