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The answer is like Jeff says but if there are two holes then you will gain little suction therefore you will need to cover the one hole (with your finger) whilst sucking with the vacuum cleaner through the other. It's best to re-seal the holes afterwards with the appropriate plugs for acryllic windows and have found a company that supplies these plugs in different sizes so you will need to measure the diameter of the holes first before ordering.

http://www.plasticsfasteners.com/WindowPlugs.html

On our caravan, many of the trapped dead flies are too big to be sucked back out through the small holes they went in through. I guess some grew fat by eating each other. I figured what I might do is drill a new bigger hole, say 5/8" diameter  (for which clear plastic bungs are available) near the bottom of the inner face of the window, and hoover them out that way. A hole that size also allows scope to reach in with a wire if some need dislodging from their grave. We have them in large numbers in all windows (by courtesy of previous owner removing the small bungs to ease condensation and not putting them back).