There's so many ways to do things. David's way sounds good. You could also create a permanent graphic still for this effect. You could bring a black solid onto Track 1 and then put a White Solid on Track 2, just so you can see what you're doing. Then go into Event Pan Crop on Track 1, click Mask, and using the Rectangle tool, draw a box to the shape you want it. Then you can adjust the opacity to your liking on the opacity setting. Then get rid of the white solid and save a ,png file of the translucent box,. Then you will have a permanent graphic you can manipulate and stretch on the timeline for however long you want the effect to last. You could also go ahead and just add your text on top of the rectangle and save that out as a .png. Once it is saved as a still like that on its own clip or media, then you can apply transitional effects to it, like a Push In, Left or Slide and then the graphic will slide in from the side. You would add the transitional effect to the front of your graphic clip, and place it over the main track. Add a swoosh sound too and you've got the whole thing going. Often Vegas's built-in transitional effects provide smoother animation than if you were to key frame a motion yourself.

Thanks for the replies. I wasn't too familiar with masking (it's been a while since I've needed to do this kind of work on Vegas), but David way seems to be working well. I'm able to duplicate the elements and change the size of the text box as needed using the masking region draggers.