Are you trying to copy 8 bars that are in the middle of the project and paste them somewhere still in the project timeline? Or at the end?
Do you know how to highlight the section and copy it?. Assuming you do, you then need to work out where to paste it. Assuming, again, that it's in the project timeline somewhere rather than the end, find the spot where the add is to start, split (S on the keyboard) the project.... shove everything on the right side of the split further to the right than 8 bars length, then paste your section in, and manually push all the bits back together.
Do all this selecting ALL your tracks so you're doing the same thing to all of them....
And find some of the many, many Sony Vegas tutorials on YouTube and start watching... lots of good basic stuff there with live screens to show you how to do it....