xna - How can I derive force vectors from velocity vectors?

I'm making a 2d shooter ala Geometry Wars. I've got my own simple physics at work driving the background grid and all my entities. To move anything in the world I apply a Vector2d force to it. The 'engine' calculates the resulting acceleration and therefore the velocity.

I am trying to port some code I found which implements the classic 'Boids' flocking algorithm, but the code I have works by calculating the Boids' velocities directly, so If i use it as is, it bypasses my physics engine. How I can translate the velocity vectors into force vectors that I can apply to the Boids and which will result in the proper velocities via my physics engine.