in idle when all is warm and running nicely I have 18 in vacuum.
When I put the car in "D", the vacuum drops to about 13 and stays there.
Is that normal or do I need to search for more leaks?
As 'idle" rpms drop so does the vacuum generated. This is not saying there are no leaks. You need to hear from someone with a good running auto trans. Mine are all manuals. Lots of things affect vacuum. If you got 18 inchs at no load idle (900 to 1000 rpms) and the vacuum gauge needle (AC off, trans in neutral) is pretty steady then you are in the desired range of 18 to 22 for a 4 cyl
When put into gear, there is an extra "drag" or load on the engine from the torque converter. IAC opens more to compensate for the extra load = lower vacuum. How much lower? I dont know if your drop to 13" Hg is typical or not.