I found a leak in the large opening window, the one above your "leak." It turned out the leak was from the vista view window above that one. Water tends to run along the outside of the inside skin (inside the shell) and come out in the darndest places.
You might have the same problem--a leak from somewhere above where you think the leak is coming from. My first bet is that it isn't the frame, so you don't have to drill out the rivets. It's the glass. Even if it is the frame, you can get some vulkem from vintagetrailersupply and do a small bead around the periphery.
If your window is double pane, you can remove the inner retaining ring and pull the whole thing apart from the inside without taking anything else apart. Sorry, don't know about construction of the single pane windows.
I decided the double pane vista views are more pain than they are worth. I've got my inside skin off and will take out the window frame and patch over it (in an attractive way).