Ummmm...I don't understand about the vacuum part, as there must be an air vent in the collection cup for all skimmers. I've built many beckett skimmers, I have a very good understanding of how they work. In that case, you have a high pressure pump pushing water into the skimmer, and the gate valve regulates the height by adjusting the back pressure which is equalized by the water height in the skimmer. The only possible difference I see here is that the main pump is simply recirculating water within the skimmer, so the feed pump is responsible for filling and maintaining the level in the skimmer. Since this pump is not a pressure rated pump, (sometimes just gravity from tank) it kind of makes sense that the level would be best maintained by a "leveling" device such as the durso arrangement. Certainly if it's your intention to submerge the output in the sump you need the siphon break that the durso provides, as opposed to just a riser. That doesn't have anything to do with rising and falling levels in the skimmer, such as when you have a typical durso in your overflow with no vent, but instead is because if you submerge the output and have no vent, the skimmer level will simply fall to the level of your sump through siphoning.
It doesn't matter that much to me as I can easily install a durso after the fact, but I thought someone would know for sure what happens with a recirculating skimmer and a gate valve with no riser. Thanks for all the advice so far!