Steve, Any VCA you like is suitable.
The surrounding component differences between the 2151, 2150, 2181 and 2180 are all associated with low levels of distortion, and the Turbo sidechain is not concerned with them.
In fact the VCA configuration used in the Turbo is precicely the same as on the main GSSL board sidechain, so whatever VCA you used in the main sidechain is perfectly suitable for that... In fact if you think about it, most people don't worry too much about which they use for the sidechain because even an unthunkably massive distortion of 1% (to quote a cataclysmic example) means an envelope difference of significantly LESS than 1%, and the sidechain VCA is only responsible for generating a 'loudness envelope', so feel free to use whatever you have lying around.
I've happily tested 2150A, 2151 and 2180 in there, and all produce compression curves which overlay each other precisely.
Having said that, for the ultimate in restful sleep, I think that if you use the same type of VCA which you used in the sidechain position on the main board, it probably 'feels' like they should track the best that they possibly could.
Good luck with the build!
Keith