I am "playing" with that staff for a while, so several comments (some I have already written before, but it is hart to find where exactly), the following is quite "technical", but I do not know how to explain that simpler:
1) Implemented in Sonar DX based MFX processing is a bit different from VST with MIDI processing. They are working with the same "material" but different way.
VST (at least in VST2) is strictly audio oriented. That means it process ( output / forward ) MIDI events in discrete audio buffer units. So before VST is asked to process/output next audio buffer size number of samples, it receives MIDI events targeting that particular time period. Either it can output MIDI at the same time is a good question, the standard is unclear there and in examples they are not doing that. But if you output MIDI during audio output, the output is "delayed" by the buffer size. Can be quite significant, especially with several plug-ins in a chain. At least that effect is clearly audible in Sonar with buffer size over 512 even with one plug-in.
DX is prepared to work with "MIDI Only". It has 2 queues, one is "Life" and another is "Recorded". In the second case (when you play MIDI from track) the events are "preprocessed" to form complete MIDI "messages", like Note with duration, RPNs, etc. Also they are sent to plug-in covering much longer buffer (there is such Sonar setting, if I remember correctly it is about 200ms). Since the whole processing is "filtering", there is no delay.
So, VST wrapper is going to be a VST host which imitate audio processing "speed" for the target plug-in. Not impossible, but not trivial either.
2) DX -> VSTi route and MIDI routing in general in Sonar is so to say "not perfect" (
read extremely buggy). DX and VSTi concepts are disjoint. I guess rewriting it completely is quite some work and just "touching" it can collapse somehow working current schema.
3) writing MFX Dx plug-in is not so hard, I think the problem is quite small audience (for example as I see for my MFX Velocity plug-in).
4) Lua based MFX processor (as requested by wst3 in another thread) is in active development. There are such projects in VST format. I have not found the same as DX. There will be no GUI, but creating such effects as CCStepper should be manageable within an hour for anyone who can calculate the sum of numbers from 1 to 64 using Basic/Excel/Any other computer language. So, for enthusiast they will be some toy to experiment with