A vacuum will offer you no advantage in cooling your aluminum block, ceteris paribus. In a vacuum, the only method of heat transfer is radiation. You will lose the benefit of any of the other three methods listed below.
There are four ways for an object to lose heat:
(1) Conduction - Thermodynamic energy is transferred through physical contact with another object.
(2) Convection - Thermodynamic energy is carried away by a moving fluid or moving gas.
(3) Radiation - The object loses thermodynamic energy via photons in electromagnetic waves.
(4) Advection - Movement of a heat-carrying substance from one location to another. For example, if a heat-absorbing substance were suspended in liquid that flowed around the aluminum block, you would have advection within a convection current.
When you cool an aluminum casting, you want to aim for uniform cooling which is not too fast. If you cool the casting too quickly you risk hot tearing, which are cracks that appear when a region of the casting shrinks more rapidly than an adjacent region. Other defects may appear if the cooling is too fast.
The Handbook of Aluminum (scroll down to Section 5.2 on page 542) says that there are three distinct stages of shrinkage during cooling: Liquid shrinkage, liquid-to-solid shrinkage, and solid shrinkage. From your question, I'd guess you are concerned with controlling shrinkage only during the last stage, when the solid block loses heat to the ambient temperature.
All aluminum shrinks when it cools. The amount and quality of shrinkage depends on the specific alloy used and the design of the casting. You have only a limited ability to change the cooling time, as you are constrained by the shape of the aluminum block. If the exterior cools more rapidly than the interior, you could stress the corners and edges of the block, as they will pull away from the still-hot interior.
You might be able to achieve an ideal cooling time by varying the speed of airflow around the block, or by leaving the block in the casting medium and cooling the casting medium, or by placing the block on a suitably heat-absorbing surface and combining this with airflow.
What you need to determine is the rate of conduction from the interior to the surface of the aluminum block. Then, you need to match that rate with the rate at which heat is carried away from the surface of the block by the total contributions of each method of heat transfer that applies. This should give you the optimal cooling time.
