Memgrove and Roadrunner,
On the Vista View windows, we have a 1979 and the inner pane is glass on ours so the 78 could be that. From what I have read, the ones with plexi on the inside have a frame that can be opened to pop out the plexi and clean between panes, putting the plexi back in. Ours was the sealed type of frame so no way to open and remove the glass in one piece.
I did remove our glass inner pane by breaking it out and used a gasket from Can-Am RV to replace the gap inside and so far, 2 windows broken and a few rains later, all is leak free. I put a little more details on my blog and photos of before during and after if you want to check them out: