Sorry for the late response (was on vacation).
Yes, VST.NET can be used to manipulate Midi messages, although I know that Cubase (and other Steinberg Hosts?) is very particular about what a midi plugin is. Most other hosts should be ok. You should check out the samples (downloads or source code tabs),
it comes with a midi mapper plugin that would be a good place to start.
The VstTimeInfo structure can be used to ask the host specific time info. When you make the GetTimeInfo call you specify, what specific time attributes you would like to know and the host reports back (in the VstTimeInfo structure) what properties were actually
filled. You need to call the GetTimeInfo method only once in your Process method.
Hope this clears things up a bit. I would suggest you start with the Midi Note Mapper sample and try to load it into your Host. When that works (means that your VST.NET installation is good), try to change the sample code to detect the repeated notes.
Good luck and have fun!