(Edited: this advice is based on experience with my 300D, YMMV)

Alternatively, undo the cooling fan (four 10 mm screws) from the water pump pulley (crank the engine over to get the screws up into position where you can get at them), pry off the two clips which hold the shroud to the radiator, and if careful you can get the cooling fan and shroud out without having to disconnect the top radiator hose. With the fan and shroud out, I found I had enough room to get at the Allen bolts which hold the pump on, and it is possible to wiggle the pump out through the PS belt without loosening it. A couple of the Allen bolts were on sufficiently tight that I had to use vise-grips on the bolt-head, and then vise-grips-on-the-vise-grips, to break them loose. There is a T-handle 5-mm ball-head Allen key, commonly used in bicycle shops, which I found to be highly useful in getting the Allen bolts off and on.

Those four cooling fan screws can be tough though. I suggest you break them loose with 6" needlenose vise-grips, before putting a 10 mm open-end wrench on them. Otherwise, unless you have a very high-quality open-end/box-end wrench such as a Thorsens, there is a high probability of rounding them. You need to pry the cooling fan forward a little, to get it off its shaft. A knife blade between the fan's flange (through which the screws go) and the pulley, works, but be careful not to puncture the radiator. Spray a little Liquid Wrench or WD-40 in between the flange and the pulley first, to lubricate the shaft.

Completely agree with the advice, to first use Gunk Engine Brite and then hose off, or else use a pressure washer, to clean the grease off the area around the vacuum pump first. Just don't let water remain very long on the water pump body, it will rust. Blow it off with a leaf blower.

Don't forget, when you reinstall the vacuum pump it takes a gasket. The overhaul kit which MB dealers sell, does not include the gasket, you must buy it separately. I put a thin, even coat of Permatex Hylomar on both sides of the gasket before reinstalling my vacuum pump in April, 2011, and used fresh high-strength Allen bolts from Tacoma Screw, and it has sealed perfectly ever since. But these bolts have rusted. Next time I'll use stainless.

The pump cover's original flathead screws are of poor quality, easily deformed by even a good screwdriver, so I suggest you replace these with stainless as well. I replaced mine with 5 mm high-strength Allen bolts, so next time the same tool works on all the bolts.

If the 240D's pump cover, like the 300D's, has its own gasket, I suggest you do like I did, use toothpicks to seal it with Permatex Hylomar too. You don't need to wait for Hylomar to dry, it's ready instantly, stays flexible, doesn't creep. Really good stuff. Originally developed by Rolls-Royce. My own pump, after I overhauled it, delivered (and still delivers) one full inch of Hg better vacuum than the FSM spec.