Insects are a very different form of life to us and it's difficult to measure their intelligence and capacity for suffering. But we can see that, like most animals, they don't act on pure instinct but change their behaviour in response to stimuli in their environment. That means some level of awareness. They act to avoid predators and threats, meaning they have a survival instinct and some awareness of pain. You could argue that it is literally impossible to exist without harming insects, so why not? But you could argue the same about many other animals (eg the ones that die as a result of crop harvesting) Where veganism draws the line is the deliberate choice that causes an animal to die.

IMO there's more of a case for eating sessile bivavles eg oysters, than insects. We can only infer, but being immobile like plants suggests they don't need to possess the cognitive capacity of animals that move around in search of food/to avoid threads. Unlike insects they are already widely-eaten (less of a barrier to acceptance). And they are high in many of the nutrients the vegan diet is low in, which might improve adherence.