Title kind of says it all, the Play VST always sounds when I use my midi keyboard even when I've got a different midi track selected.  I tried turning midi echo off and on, no difference.  Also, it's not just one VST but many.
 
For example, I just incerted a new instance of Superior drummer, still doing it.
 
Any ideas?

Could be a routine issue. If this is in SONAR X3, there has been a change in default behavior for synth plug-ins. As of X3, synths have "Enable MIDI Output" enabled by default if they can send MIDI data. If instrument and MIDI tracks are set to Omni, they will pick up MIDI data from plug-ins which have "Enable MIDI Output" enabled

But then why would it only be this one track doing it, and it only started half way through the project.

Kroneborge
But then why would it only be this one track doing it, and it only started half way through the project.

I've found it kind of depends on what you have selected at the time, I've had drum tracks play other synths seemingly randomly before. Check all your VST's and turn it off if it's on (you should be able to right click on them to see the option), see if that fixes the problem.

nope, they don't have midi ouput enabled, in fact I can't even turn it on.

did u try in the synth options on the synth rack  'Force midi echo thru current track'  uncheck?