That depends on who you talk to. Definitely, if you want a stock to be nice and clear, you don't want to boil it as the movement in the water can break up the vegetables enough to make it cloudy. In my experience, usually the stock is kept 'ticking over', meaning you have a tiny bit of momement in the water but it is barely a simmer.
Some chefs I've worked with insist on keeping the stock at a constant 90-93 degrees celsius but personally, I prefer to keep it at a very gentle simmer.
Regarding a vegetable stock specifically, If you use quite a high proportion of leek (cabbage can work well too) you can keep it at a gentle simmer and it will thicken very slightly. This is good to use as a 'nage'.
At the end of the day, the end use will dictate how you treat your stock.
If the Sous Chef wants it boiled, he's the boss, but as long as you have a good understanding of the subject, you can make a good decision when you need to.
Happy Cooking!Kiwisizzler’s Blog