This can be done a couple of ways and which way really depends on the most appropriate blend to best support the image. You can either gradually fade a track to a lower volume using a volume envelope, or discreetly have it drop on a dime using the same tool.
Let's say I want to have the music drop 6 dB but not completely cut out so I can hear the dialogue track more clearly. In Vegas Pro or Vegas Movie Studio you can use a volume envelope to change the level at particular points. When engaged there will be a blue line in the middle of the track at 0 dB, clicking on the line will create a point, so you want to click twice, once for your reference point and next for your change point and add two points at the end of the drop when you want to bring the track back to volume. Note, you can use this tool to make instant drops or gradual fades.
Alternately, split the track where you want the sound to drop but not cut out. On the left of the split is the music at "normal" volume and on the right is where you drag the volume down to the level that works with the dialogue. The following links will describe the Volume Envelope tool in Vegas.
This one is a bit longer but covers a lot more including the 'pan' tool: