Two ways. The simplest is just restore the envelope to zero (the middle position). If your hover you mouse it reads out where it is currently set. The second way it just drop the event to another track (insert track first). You are unlimited in the number of tracks you can have. Just remember if you have one track on top of the other unless you adjust the opacity of the upper track all track below it can't be seen. For audio it doesn't matter, each track can be adjusted seperatley and you can set different filters and have different volume envelopes for each audio track you have or go to the next level and add bus control.

Also Vegas works on the "event" concept. You can apply effects either to your whole video, an entire track or by an event. You can split the timeline anywhere into as many events as you want. Move you cursor over the timeline where you want a break, click, select EDIT form the menu bar then select split. Dragging the right edge of the event exposes more of the video, dragging it left further reduces it. This is non destructive and does not effect the source file.

You really need to do the basic tutorial to learn some of the basics.