The relationships require you to select the child value, even if there is only one option. There was a time in 5.2 with the optional web portal you could install, that would select the child if there was one present, but that has gone away. What I would do is use a vRO workflow in the building machine state and have it set the CustSpec value based on the property that drives the value of the others and pass that back to vRA. It is a pretty easy operation and a good opportunity to play with vRA -> vRO if you haven't done anything with it before.
I wrote a blog post on how to update a property in vRO and pass it back to vRA, there is a follow-on post that has a more complex update that allows you to have a generic blueprint that updates the cloneFrom and cloneSpec properties based on what OS is selected.
http://www.realize.net/2014/10/vcac-6-1-update-custom-properties-vco/