Answer: The velocity vector is perpendicular to the acceleration vector; the acceleration vector is parallel to the net force vector.
Explanation:
In a uniform circular motion, the meaning of "uniform"is the same as for an uniform straight motion, i.e., the module of the velocity vector (its speed) is constant.
Now, if the object were not describing a circular trajectory, it should move at constant speed, in a straight line, provided no external forces acted upon it.
If there is an external force acting on it, making it to follow a circular trajectory, this force doesn't change the instantaneous value of the velocity, but it changes his direction instead.
While the direction is changing, it always keep tangential to the trajectory, due to if at any moment the force disappears, the body must continue in a straight line at constant speed, following a line tangent to the circle.
It can be showed, that the acceleration vector, defined as the change in velocity over time, always aims towards the center of the circle, and is perpendicular to the velocity vector.
As the only net force acting on the object (assuming a horizontal trajectory), is the one that causes the acceleration, the acceleration vector has the same direction as the net force.
