The setting doesn't need an FSX restart so, regardless if there's a problem with saving it, you should see the effect as soon as you change it. Note that setting will only control dynamic ground vehicles created by the scenery itself, default ground vehicles created by FSX will be controlled by the Ground traffic slider in FSX.
About your problem with saving, I've checked it and it correctly saves the setting so, it's possible that some other program in your system is keep the FSX.CFG open for writing, which will prevent the Addon Manager to save that setting.