Box2d - Variable length array of non-POD element type 'b2Vec2'

You have to create the array on a heap:

b2Vec2 *array = new b2Vec2[count];

Don't forget to delete the array manually when finished.

or better use std::vector:

a)
std::vector<b2Vec2> vertices(count);
vertices[0].set(2, 3);
vertices[1].set(3, 4);
...

b)
std::vector<b2Vec2> vertices;
vertices.push_back(b2Vec2(2, 3));
vertices.push_back(b2Vec2(3, 4));