Recently I've started to roast vegetables without adding oil in a bid to eat healthier. However, they end up coming out of the oven looking more dry instead of crisp and browned, with not as much of that roasted flavour. Casual googling has lead me to the Maillard reaction, but is this reaction affected by the amount oil covering the food?
The Malliard reaction is quite complex. The article I linked defines it as
many small, simultaneous chemical reactions that occur when proteins and sugars in and on your food are transformed by heat, producing new flavors, aromas, and colors.
Oil does not necessarily need to be present, though, especially with regard to meats, fat is often there. If you scroll down to the comments, someone does ask the author about oil and marinades. He replies that "fats, under high heat, produce their own separate browning reaction."
Maillard is not a reaction of fats, but the browning of fats works in tandem with the Maillard reaction to produce flavors and aromas.
Oil does promote caramelization of vegetables in a roasting situation, which is what it appears you are missing. It doesn't take much for the desired effect.
Butter, specifically the milk components (sugars and proteins), turns brown and flavorful when cooked. Clarified butter doesn't have nearly the same effect.
Fat on vegetables also slows evaporation while roasting, leaving more moisture in them.
Oils will also get hotter, than evaporating water, which allows the vegetables brown more.
A little bit of pure olive oil can go a long way to all of the above points.
I'm pretty sure that I'll is not part of the Maillard reaction, which by definition happens between proteins and carbohydrates. But remember, browning is much more than only a Maillard reaction.
I can confirm your observation that oil creates a better texture on roasted vegetables, and if I had to guess, I would say that it turns the roasting process slightly in the direction of frying, which accounts for the nice surface through the typical processes there (different rates of heat transfer, different starch gelation, different amounts of steam escaping, etc.).