Your mental model is correct: vCPU means hyperthread (except for shared core things like the f1-micro, g1-small, etc.).
We had a different measure of "relative performance" called GCEU (GCE Units) but stopped publishing that as it's pretty meaningless for most people. We do our platform qualifications at Google to ensure that for users that "don't care" which CPU platform that they're on, that they get improving performance/$ and so on. But for GCE, we clearly document instead the platforms and base/all-core/single-core frequencies we use [1].
tl;dr: if you want to choose your processor, stick with N2/C2 and our upcoming AMD machine types. If you're okay with us deciding for you and want a big discount, give E2 a spin!
