Toyota Vitz rough idle

Let's assume you have already checked for small vacuum leaks in the rubber tubing on the intake manifold and replaced the PCV valve (if applicable) . . .

I had the same problem on three different cars (two late-model Mercedes-Benz). The problem were due to dirty fuel injectors, spark plugs, intake valve stems, idle air control valve and flow ports, and other deposits. Cars are particularly susceptible to deposits in cold climates, like Ireland.