I would put the oven at 400 or 425F. That's what most recipes call for. A lower temperature will definitely promote dryng. I would be reluctant to add much sugar, including balsamic vinegar, for fear that it would burn.

I'm still betting that you aren't using enough oil. That's your "sauce." We will drizzle good oil at the table too, but the oil you roast the veggies in will acquire some of their flavor.