Parts for my old rode
 

I have a Viscount Aerospace Pro that I've had and ridden since I bought it new in 1977. I ride it now when I met up with the kids and grandkids at camping events. The problem is that the rear derailed has worn out and doesn't keep the chain tension. I need to replace the rear derailed but it is a ten speed with 5 rear gears on the frewheel and I can only find a 7 speed derailed. Any one have any ideas

Welcome to bike forums.

7 speed derailleurs are a good replacement for 5 speed derailleurs on bikes with fiction shifters. What derailleur did the bike have originally?

Agree that a 7speed RD of some sort should work.
Was your Viscount subject to the fork recall? https://www.sheldonbrown.com/lambert.html

Your Viscount has friction shifters, so the "speed" rating of the derailleur doesn't matter. IIRC, the Aerospace Pro came with a Shimano "Titlist" rear derailleur, so any medium or long cage derailleur ought to work.

N.B. If your bike still has the original aluminum fork, you should replace it with a steel one, just to be safe.

Fellow Viscount owner here. Your Pro model was equipped with the Shimano Crane in the rear and Titlist on the front. If you want to keep the bike original, they are always showing up on eBay. The Aerospace GP which was a notch down from the Pro has the Titlist rear. We have a dedicated forum at Home | Viscount & Lambert Bikes

Unless keeping it original means a lot to you, find a nice used SunTour Cyclone rear derailleur, or even a SunTour V.