How Not to Lose Liquid in Vegetable Stock
If you uncover the stock, liquid will evaporate. For maximum yield, keep the stock covered. There are no downsides to this as far as I know.

The leftover vegetables are likely very completely mushy and flavourless, depending on how long you have cooked the stock. They have given their flavour to the stock. Any vegetables with flavour left you can use however you want (although their texture might still make them less than desirable). If not, composting is an option.

Regarding cooking time: if your stock tastes good (I like to add a small pinch of salt to a spoonful of stock to taste), and most of your vegetables don't, there is no point in cooking the stock further.