How Not to Lose Liquid in Vegetable Stock I just made my first vegetable stock (I chopped everything roughly and started by sauteeing the onions celery and carrots a bit, added a lot of veg like celeriac and leek tops, then I added 3 liters of liquid, brought it to a boil and simmered it uncovered for 1.5 hours.) 

I feel like I only have 2.5 cups of stock after and am upset. It's a waste.

I simmered it uncovered. Should you simmer it covered? Any tips for not losing so much stock liquid? Also, do you use the leftover stock veg for anything?
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.