Like a bicycle pump, vibratory pumps use a one-way valve to build up pressure. This diagram makes it clear how it works:


Image courtesy of Made in the GARAGE (water flow is left to right)

Pressure builds fairly slowly while the vibe pump piston goes back and forth because of their relatively low flow rates. In contrast, rotary pumps have a far greater flow rate at espresso brew pressure range, so they fill the chamber and fully pressurize nearly instantaneously. Some marketing literature refers to the vibratory pump's naturally slower pressure ramp as preinfusion; to put a slightly finer point on it, I refer to it as progressive preinfusion to distinguish it from another preinfusion mechanism like the E61 expansion chamber.

allon wrote:However, when the vibe pump isn't operating, I believe that it will pass line pressure...

That's true for a rotary pump because of the bypass valve, but not for a vibratory pump. For the nitty gritty details, see Pressure profiles, preinfusion and the forgiveness factor.