I would be reluctant to cook the vegtables separately. The problem is that not only do many of the vegetables give flavor to the liquid to the stew, but more bland items absorb the liquid, keeping them from being so bland. In the case of starchy vegetables, they also contribute to the body of the stew.
It's also difficult to give exact times for cooking the vegetables, as there are a number of factors:
- type of vegetable
- size and cut of the vegetable
- acidity of the cooking liquid
- temperature of the cooking liquid
- personal preference
Now, all of that being said ... I make a stew in which most of the items are cooked separately. If I roast meat and vegetables (potatoes, carrots, onions, peppers, etc.), and have a fair amount left over, I'll cut up the meat, start it cooking in a pot with some extra liquid (the meat juices, maybe some stock, a can of tomatoes, etc.), then add the roasted vegetables and a grated potato (to replace the starch needed to thicken it), and if I have them, some green beans or peas. Let it all warm through and the grated potato disappear, and you can then either serve or let sit in the fridge for a day or two so the flavors meld.
Based on that, I'd be much more likely to use roasted vegetables before boiled ones. You concentrate the flavors, rather than washing them away. If you use a roasting pan that you can deglaze, you could even rescue anything that leaked out and get it back into your stew. If you roasted each vegetable seperately, you could get the control that you're asking for to ensure that everything's cooked as you want it. (but I'd still give them a little bit of time to mingle)