Yes, there is. It's just not what you're thinking. There's a pressure differential between the atmospheric pressure in the fuel bowl (and accelerator pump fuel source) and the air going past the squirtor nozzle. It's not engine vacuum as you might normally think of it, but there is a difference, and the fuel in the circuit is higher than the airstream going thru. It's the same "vacuum" that opens the secondaries in a vacuum secondary carb.
In regard to vacuum at WOT... In fact, many engines do pull vacuum below the throttle blades at WOT.