In my opinion, the 1980s synths with their purity and cleanliness and stereo sound and etc. are probably less difficult to emulate with software. They, to me, are less distinctive in comparison with modern synths because of a lot of "improvements" like pitch stability (DCOs, I'm looking in your direction), precision, stereo sound, reduced noise, more ICs, etc. While they still have a great sound, they're just too damned improved to generate the sort of aural differences older synths do in comparison to software, emulators, etc.
So: all of the suggestions I am about to type are in regard to making your computer sound vintage vintage, not just 1980s vintage.
1. Let go of stereo. Most analog synths weren't in stereo. Recording a supercool pad patch in stereo is a bit of a giveaway. Record in mono. If you want to pan your synth in the stereo field, that's okay.
2. Make your oscillators misbehave. That desirable analog sound comes from drifty or imprecise (or both) oscillators. Do something to make your oscs drift... like a very very slight very slow pitch LFO.
3. Let go of resonance When all the great synth players were famous for playing all the great synths, they (in comparison to now) didn't really crank on the resonance and worship the filter sweep. Synths in the 70s were mostly NOT resonancy. Learn to embrace the delight of a filter envelope which ISN'T sounding like a TB-303. Discover a time when resonance was a subtle effect and not the ON OR OFF effect it is now.
4. Distort The delightful frothy yumminess often called "fat" has to do with distortion... it is equally present when the resonance is off, and the notes aren't deep. You get it by slightly overdriving your oscillators or filters (or, at least, that's where Moog is getting it from) It's a difficult thing to fake unless you're a programmer unless your softsynth has the ability to modulate oscillators and filters with HFOs (or, audio-range oscillators). If your softsynth has that capability, use it! Apply HFO modulation to your filters or oscillators or both... but judiciously. The point where it becomes distinctly noticeable is too much!
5. Learn to live with noise. Or, "Distort, Part 2." A good way to add fat is to run your synth through a tube amp or tube preamp. Find an overdrive setting where it doesn't sound overdriven, but isn't NOT overdriven, either. Yes, this will add noise... but you WANT noise. Vintage synths were noisy and hissy and hummy. If you want vintage, you WANT noise.
6. I know you want to beat me for this, but don't sequence. Was sequencing occurring in the 70s? You betcha. However, sequencing overtook the world in the 80s... and has continued up until today. As a result, a sequenced synth sounds modern, even if it is a Moog modular. Try to avoid sequencing if you can... your (hopefully) slight imprecisions will trick your listener's ears. (by sequencing, I primarily mean quantizing)
7. Listen to vintage synths. Do NOT listen to recordings of vintage synths buried in with lots of more modern synths played in modern genres... it will not help you get a vintage sound. Listen to vintage synths in their natural vintage habitat... even if that means you have to listen to music you're not into. It will help you understand the differences between what vintage synths sounded like when they earned their following, as opposed to what they sound like now.
8. Avoid using functionality not found on vintage synths. Research the functionality found on your favourite vintage synth, and don't do anything with your softsynth which that synth couldn't do.
"I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question." -Charles Babbage
"Unity and Mediocrity are forever in bed together." -Zane W.