Are vacuum pump effective to improve wine conservation?
Well, I can definitely point you in the right direction towards a Cook's Illustrated test of vacuum wine keepers. Unfortunately, I don't have a membership to their website, but this free portion of the article suggests that at least some of them do in fact work better than just replacing the cork.

For our wine stopper test, we evaluated several methods of saving leftover wine, ranging from pricey &quot;wine preservation systems&quot; and a much simpler (and cheaper) tool: an old water bottle. We went to the kitchen to test them all. First we drained 10 ounces of red wine from six identical bottles, sealed them, and then refrigerated the bottles for 10 days. At that point, we compared them all with a fresh bottle.
Tasters found little difference among the bottles. All but one kept the Merlot perfectly quaffable. Even a small plastic water bottle with an airtight, screw-top lid proved effective. It did have one drawback—you must fill the plastic bottle completely to eliminate air.
Following the taste test, testers limited their criticism to design flaws, most notably for the high-maintenance costly model and the model with the perplexing stopper which offered no written directions. Our winner proved to have a strong seal, was easy to use, and had a bargain price tag. In addition, its trademark click indicated when a vacuum seal has been achieved.&quot;

EDIT: Cook's Illustrated, America's Test Kitchen and Cook's Country are now offering all three websites for one price, so I re-subscribed. The winner (by a large margin) of the test was this one: VacuVin