Systems and methods associated with providing a concentrated product gas utilizing a vacuum swing adsorption process are provided. In one embodiment, the system includes first and second sieve beds, a scavenger pump, and a controller. In one embodiment, the method includes: separating one or more adsorbable components from a source gaseous mixture via first and second sieve beds in alternating and opposing filling and purging cycles of a vacuum swing adsorption process to form the concentrated product gas and pressurizing the concentrated gas for delivery to a user. Other embodiments include selectively controlling a variable restrictor based at least in part on a desired output setting to selectively provide flow between the first and second sieve beds such that the flow for at least one output setting is different from the flow for at least one other output setting in relation to corresponding filling cycles.