I'm working my way through learning vacuum technology.
I have a 10l vessel which I want to evacuate to < 10mbar. I'll use this diaphragm pump.
Do I need to add a (manual) venting valve to the system, or will switching off the pump do the job?
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I am not a vacuum expert, but if you look at the technical data for your pump, you'll find this "Leak rate 6x10-3 Pa m³/s". I take this to mean that your 10 liter = 0.01 m$^3$ vessel will leak at a rate of 0.6 Pa/s. That's pretty slow. You'll want a manual venting value.
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You will likely want more than just a vent valve - to avoid contamination of a vacuum chamber, typically you re-pressurize with an inert gas like nitrogen to avoid sucking condensates and water vapor into the chamber. Every time you add some condensate to the chamber, they'll need to desorb - which is an extremely slow process compared to pumpdown.
