(disclaimer: I know nothing about the vectorization code)
Now, is the vectorization code really dead code or it is code that needs love to come back to life? By removing it from the code base, you are probably sealing it's fate as dead code as we are limiting new or existing contributors to act on it (even if it's a commit hash away). If it is code that needs love to come back to life, grep noise or conditional compilation is a small price to pay here, imho.
As a compromise, is it possible to move vectorization code into it's own submodule in git or is it too intertwined with core GHC? So that it can be worked on independent of GHC?