I've been using Vivaldi to experiment with various techniques to improve the ensemble sound of solo strings. The results are in this playlist.
The main techniques are:
1.) Time spreading the Note-onsets.
For example, the 6 first violins are individually delayed anywhere from -50 milliseconds to +50 milliseconds (negative delay means the note starts before the beat). Each of the 8 'players' gets a different delay on each note. The overall spread from earliest player to the last is greater for the first note after a rest, as it usually is in an ensemble.
2.) Detuning the Note-onsets.
Each 'player' begins their note anywhere from 30 cents flat to 30 cents sharp, and quickly slides into tune. The amount of detuning is small for small pitch intervals (2 or 3 semitones) and greater for big pitch intervals.
3.) Detuning the Note Sustains.
Even with the above two techniques in place I found that sustained notes still tended to fuse into a single sound. Adding vibrato helped (at a slightly different rate for each player) but even better was adding a degree of random detuning, -10 cents to +10 cents, separately for each player, changing constantly at a slow rate like a random LFO. The detuning amount is updated every second or so, again different for each player.