Q&A

First, needing an oil change should not influence seeing smoke or steam from the defroster.

This sounds like your heater core sprung a leak and you saw the steam releasing from the crack, or the spot from which it's leaking. Did the 'smoke' have any particular type of smell? Did it have the sweeter smell like coolant or did it smell like burning electrical/plastic?

Does the passenger side floor feel damp to you? If it is damp, how does it smell?

If adding coolant led to the smoke from the defroster vents, this really leads me to think your heater core is the culprit. hope this helps!

There are so many things that can cause rough idle, can you give more information? Did the car recently overheat? Is there any fluids leaking on the engine? Are you getting any smoke or steam from the exhaust? When checking for steam, remember to wait until the engine has run for awhile as condensation can build up in your exhaust during the night. If you cannot tell if it is smoke or steam, hold a blank sheet of paper behind the tail pipe, if it is oil smoke, then oil dots will show up on the paper.

Some things you can do and check;
Check the spark plug wires, make sure they are tightly connected and not chaffed, broken or burnt.
Check your PCV valve for excessive oil or clogging, (it will be in a rubber housing or hose leading from your valve cover to your throttle body/carburetor)
I don't know the engine setup in that model, but if you have a distributor cap, check it for cracks and wear, also check the rotary cap.
Check all rubber vacuum hoses on the engine for cracking.
If you can afford it, change the spark plugs, plug wires, rotor and rotary cap, these parts all need to be changed over time and your car is getting older now.

I have a 1988 Chevrolet Celebrity. Around 140,000 miles or so, it began to idle rough. Also, around certain speeds the engine appeared to be causing vibration which was detectable in the steering wheel.

I took it to my father's mechanic. He found out two things - 1. The engine mounts needed to be replaced. 2. Firestone (where I had last had work done to my car) had re-connected the spark plugs in the incorrect sequence. Getting the engine mounts replaced took a few weeks, partly because of the difficulty in servicing the Celebrity's engine. (It is kind of crowded under the hood.)

After having the mounts replaced (and the distributor connected properly) my car ran amazingly smooth. I had not felt so little vibration from it in ... well, at least a year, possibly more.

Long story short - your engine mounts may be old, cracked, or otherwise in need of replacement.

So far as your experience with coolant and smoke, I have no advice.