Stack overflow error c++ after vectorization

I get a continuous stack overflow error when trying to test this little program to test the performance difference between Structure of Arrays vs Array of Structure. I'm clearly doing something wrong, but I have a really hard time figuring it out. Is anyone able to help?

#include "stdafx.h"

#include <vector>

#define NUM_ITEMS 10000000

struct Vector3 {

    Vector3() : x(0.0f), y(0.0f), z(0.0f) {}

    float x, y, z;

    Vector3 operator+=(const Vector3& v) {
        Vector3 v_new;
        v_new.x = this->x + v.x;
        v_new.y = this->y + v.y;
        v_new.z = this->z + v.z;

        return v_new;
    }
};

struct SoA {
    SoA() : pos(NUM_ITEMS), vel(NUM_ITEMS) {}

    std::vector<Vector3> pos, vel;
};

struct AoS {
    Vector3 pos, vel;
};

int main()
{
    std::vector<AoS> entities[NUM_ITEMS];
    for (int i = 0; i < entities->size(); i++) {
        entities->at(i).pos += entities->at(i).vel;
    }

    SoA entityManager;
    for (int i = 0; 1 < NUM_ITEMS; i++) {
        entityManager.pos[i] += entityManager.vel[i];
    }

    return 0;
}