My wife’s 2002 Mitsubishi Lancer (automatic transmission, 2.0L engine, ~81K miles) idle is dropping to ~500 rpm whenever you come to a stop. It recovers fairly quickly, but sounds like it’s going to stall out just as you come to a stop. Is it worth trying simple things like plugs and wires / cleaning the throttle body, or it it likely something more complicated and expensive like the Idle Air Control Valve?
The cheapest thing to do for this problem is to give the throttle body and the IAC valve a good cleaning with a throttle body/air intake system cleaner.
Remove the air intake hose from the throttle body, and while holding the throttle open, spray down the throttle body. Then remove the IAC valve and clean the pintle on the valve and also the idle air bleed port on the throttle body.
Put everything back together and cross your fingers. If this doean’t improve the condition, then most likely the IAC valve is getting lazy and requires replacement.
Start with the suggested cleaning and lubricate the various EXTERNAL connectors to the trottle body. No lube inside just cleaning spray and a soft plastic brush if the carbon/deposits are sticking. This could easily be a IAC that is slow. Vacuum could be an issue but you need a gauge and maybe a mechanic you trust would be a better answer. Try clean and lube then mechanic.
Aside from all of that though - if any of those “simple things” aren’t up to date as far as scheduled maintenance goes, then you should just do them. Other kinds of problems will lurk.