The key to getting the vegetables to roast at an even rate is to cut them accordingly. The potatoes will take the longest to soften, so you'll want to cut them into fairly small pieces. A good visual would be to cut them into pieces about the same thickness as the shrimp (less than an inch thick.) Cut the onion in half and cut out the core. Then cut it into wide lengthwise strips, or in other words french cut the onion. Cut the pepper into wide strips. Since they'll be in the oven roasting for a full half hour, you'll want them to be wider than normal so they do not overcook.