[karbomusic]

The basic premise is you hold out a note or chord on the guitar, which eventually comes out of the speaker which in turn vibrates the string(s) at one or more sympathetic/resonant overtones, which gets amplified again, rinse and repeat. Same process as microphone feedback only richer and much more pleasing. Actually, harmonics, overtones etc. are what guitar feedback is all about and they are near perfect sine waves which is very important. There are lots of reasons VSTs don't handle this well. If you can take care of some of those issues you can get better feedback.

Midrange vs fullrange
Everything that matters about guitar feedback is in the midrange. This is precisely why full range speakers, aka monitors do such a horrible job at it. Even a mic'd amp doesn't feedback through the PA speakers alone as well as it would the guitar amp/speaker. The guitar speaker has such a midrange response, it tends to be perfect for feedback. Running a VST through fullrange monitors, no matter how accurate are going to be lacking. The accuracy is part of the problem because guitar speakers are anything but accurate. Their misgivings actually assist feedback. A woofer and tweeter attempting to create midrange isn't the same as an actual midrange speaker.

FX
Most FX are the death of good feedback. The very best feedback you can get is with the purest of signals (remember feedback is like a sine wave which if disturbed kills the feedback). Compression/distortion are about the only FX that don't fall into this group (they actually help) but I don't call those effects. Most other effects make changes in the regularity of the signal, phase shifters, chorus, reverb; gates are obviously out of the question as well. VSTs tend to add lots of additional stuff that makes the VST/guitar sound like a "record" but those are horrible for feedback as they break that regularity of the pure signal.

Latency
Its a guess of mine but I would assume latency in the chain wouldn't help but is likely at the bottom of the list. I'm sure it affects that tight bond between speaker strings required for great feedback somewhat.

So there are a few things one can do to make it better.

1. Cut the lows/highs and boost the mids on the VST and or the master in Reaper just for the part you want feedback. Increasing the mids should make an immediate difference.

2. Remove any and all FX other than gain and compression. You might even increase gain and compression slightly for recording the feedback. Remove anything stereo or use the mono switch on the track, go all mono for the recording part. I might go so far as to say only have a single monitor on because that sine wave likely won't arrive to the string(s) from each speaker at the same time once again disturbing the alignment that allows the feedback to happen.

3. Get closer to the speaker but it all depends on sympathetic vibrations and angle of the guitar in relation to the speaker. This will greatly affect which overtones tend to feedback. Simply turning slightly in one direction will alter the feedback and make it morph into a different overtone. Also by gutting the lows/highs you can effectively turn up a tad more with less damage to the monitors and assisting in feedback.

4. If the guitar doesn't feedback well through a regular amp, its not going to through a VST. Every guitar also has its own dominant overtones that feedback easier than others.

5. Be aware of vibrato, use a single static sustained note or chord to start. The variation from the vibrato sometimes breaks that 1:1 relationship between speaker, frequency, note, string etc. It can be used to coax feedback in some instances but it works better when you already know the feedback is easy to achieve and or after the feedback has started.

Ideally a real amp is the easiest and purest way to achieve. Depending on what you have at hand you could even run an external signal to some guitar speaker where the speaker isn't actually recorded, it only provides those vibrations back to the string. However if one takes all of the above you can at least get basic guitar feedback from many VSTs but it simply isn't going to work well with most out of the box settings for both the VST and the monitoring. Boost the mids, cut the low/highs and get all the extraneous crap out of the signal and it will work better FWIW.

I still use a real amp for it, its just to cool for school but based on what I've seen most using VSTs could pull it off better if they had a little more info on how it works and why it doesn't many times with a VST. I might also add that many, if not most small studio monitors simply can't get loud enough to fuel the feedback properly.