as soon as I change my default device (like if I'm changing from my Rift to my headset or the other way around) then the mic in VA changes and I either deal with it or go reconfigure again...
Is the default recording device changing by itself when you unplug/shutdown the Rift? Is VoiceAttack running at when you do that?
VoiceAttack has the capability to change the default recording and/or playback device either when it starts or through a command, so while you can't "lock" it as such you can make sure it's set correctly when VoiceAttack is running.
The startup setting is under
->"Audio" tab->"Set Windows Default Audio Recording Device on Startup".
The action is under "Other >"->"Windows"->"Change Default Audio Devices".
EDIT: Fixed image links