You could easily do that with a simple text event -- use a transparent backgound and put it on the highest track so that everything else shows through it.
You can put anything you want containing an alpha channel (transparency/checkerboard background) on your top track then use the (left) slider to bring down the opacity to the level you want.
You can use a logo, some picture, etc... as long as you have this alpha channel.
Vegas is very "friendly" with PNG files.