Building a 12-string effect requires several different elements, each used subtly, to create an effective illusion. This can be accomplished easily if your digital audio workstation has auxiliary sends on each track. Apply your VST effects only to the guitar signal going through the auxiliary, and mix its return with the original, untreated guitar sound. Each effect should be set up to output only effect, no original signal, for best effect.
Delay is the first step, re-creating the effect of a pick striking strings at different times. Use a single repeat and a delay time starting around 15 to 20 milliseconds. Adjust this amount up to create a more obvious effect; however, only a slight delay is needed at this point. Retro Delay, Stereo Jackie and Twin Delay are three freeware VST plug-ins suitable for this task.
Four of the 12 strings are tuned one octave higher. Pitch-shifting plug-ins can be used to add this octave sound to the delayed signal, mimicking the octave tuned strings. This method will also octave the high E and B strings, which are tuned in unison; however, that is due to impractical string width and tension. This pitch shift will simulate overtones in the finished product. Agares RackA3 includes a pitch shifter in its freeware collection.
Adding a chorus VST plug-in will give a swirl to the sound. You may decide to apply this to both the original guitar signal and the auxiliary return, in which case you will likely use a gentle setting. Adding it only to the aux send lets you be more aggressive with the setting, and the aux signal will react with the original track in a more organic way. Classic Chorus, CH-1 Chorus and Cobalt Chorus are available freeware plug-ins.
A reverb VST plug-in is best placed on both original and auxiliary return signals. A bit of reverb will blend the two sounds together to sound authentically like one instrument, as well as giving the sense of space around the guitar. Try freeware plug-ins like TimeVerb, TAL Reverb or Reverber, and don't ignore the VST effects included with your DAW. At this point, increasing or decreasing the amount of auxiliary return will vary the level of 12-string effect.