A whole chicken releases a fair amount of liquid. As it falls among the vegetables, it is sheltered from the higher heat and so it lingers. This isn't necessarily bad as it can impart chicken flavor, but it does inhibit crusting as you've discovered.
A rack will help in some ways, but not totally unless it gives you 3 or more inches of elevation. It will still be less crusty directly under the chicken, but the bottoms will get crustier. This technique with a butterflied chicken makes excellent chicken and potatoes. I've not tried it with mixed vegetables in bigger chunks. I like the potatoes even more than the chicken itself. http://www.food.com/recipe/crisp-skin-high-roast-butterflied-chicken-with-potatoes-213865
Cutting the chicken up into the joint sized pieces helps some as you disperse the juice more across the roasting area.
You could start the chicken pieces alone, then add the vegies after say 10 minutes. Pull the chicken when done and raise the heat on the remaining vegies for some crusting while the chicken rests.
Roast the chicken on side of the pan. Roast the vegies on the other with an aluminum foil dam between them?