It's not necessarily a vacuum leak. It COULD be a vacuum leak, either external from a hose, brake booster, or gasket, or an internal leak from a gasket, EGR valve, or EVAP purge.
But it could also be a MCC adjustment, fuel bowl level adjustment, clogged jetting, saturated float, incorrect TPS setting, and various other causes.
It's fairly easy to check for vacuum leaks, so that would be a good starting point to eliminate that possibility.