I finished old blues and never got that achievement, which is for Recovering X-8 vertebrae-pulse-de-sensitizer frequency. is it not part of the the x8 quest?

did I miss something?

I'm not sure which one that is, and the Vault doesn't shed any light on the subject.

Hmm... is that the frequency that allows you to break down forcefields? Did you do ALL of the tests in X-8, including the ones with the Nightstalkers?