What would cause my Mercury mariner 2007 3.0 V6 to have a rough idle

Hello, since pressing the brake pedal makes it worse apparently, it does sound like an engine performance concern that could be exaggerated by engine vacuum being manipulated. The brake booster uses engine vacuum for power, so this could be a clue. I would check for any air leaks in the air intake system or vacuum leaks under the hood. Other than that, basic testing of ignition system components (coils, spark plugs), base engine condition (timing and compression) and fuel to air mixture ratios is needed. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops