Is it worthwhile to replace the glass of 20yr vinyl windows that have lost their seal? The frames appear to be in good shape

Not really. Around me it's $75 to $150 to restore each sash. You could end up repairing the fogging issue and then find the frames fall apart a few years later. You're dealing with an inferior window already, so it would be wiser to replace them and have a warranty again.

- If they're already replacement windows of stock or normal sizes with fillers and you're somewhat handy, you could very easily and cleanly re-replace them entirely for as little as $130 each and still end up with a better window and better efficiency and comfort.