A dissolve or crossfade between two video clips (called Events on the Vegas timeline) is very simple. Just place 2 events on the same track, and drag the 2nd event backwards until it overlaps the first. Vegas automatically creates a crossfade where the events overlap. The default fade type will give you a smooth crossfade.
It’s the same for audio events. By default, the audio associated with a video event will also crossfade into the audio from the other event.
You can change the video fade to other transition types very easily. Just go to the Transitions tab in the docking panel (normally to the left of the preview window) and you’ll see a browser with all the transitions. Just drag the one you want onto the crossfade in the timeline and you’ll get a dialog box that lets you tweak the transition.