To be honest, it sounds like 'Overdriven Guitar' patch from AC97 General MIDI Soundset, which in turn was based on Roland Canvas General MIDI module if my memory serves right, so it will be not an unreasonable assumption to conclude, that they sampled this particular patch of that piece of gear. Worth checking, IMHO.
Everything you hear in those old scores from games of past you can easily research. Study the gear specs and read about actual composers - in interviews and such you can find chunky details very often.
But for now - Try mixing square and saw waves with noise, somewhere around 5:4:1 proportion, add unison, set snappy attack and short release with maximal sustain and then put it trough 16 bit/32Khz degradation. Vibrato and portamento are done via note effects and articulation (trackers are best suited for that, but modwheel on conventional synth/sampler will do too), which you can easily deduce from not all the notes being played with those atributes.