Vegas doesn't really work with tempo as such. It isn't aware of what music is all about in the way that ACID is. However, there is somthing you could try that might accomplish what you want. Split the song where you want the tempo to change by placing the cursor at that spot and pressing S. Find the end of the song and place the cursor on the right edge of the clip. Hold the Ctrl key down and stretch the cilp, dragging left to speed it up or right to slow it down. You can be playing the clip while you make these changes so that you can hear the difference in real time. The default behavior is for Vegas to change the speed while maintaining the same pitch. You can right-mouse-button click on the clip and open up the properties menu if you want it to change the pitch as well. Of course, you can add as many splits as you wish.