I have installed vector-0.10.12.3 and vector-0.11.0.0. At some point, I had hidden both from the package database; however, I later used ghc-pkg expose to mark one as visible again. Now I seem to be in a strange state where ghci and ghc-pkg disagree about what is hidden:

% ghci-7.10.2     
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Prelude> :m +Data.Vector.Unboxed

<no location info>:
    Could not find module ‘Data.Vector.Unboxed’
    It is a member of the hidden package ‘vector-0.11.0.0@vecto_3jMaUrldidp1bqsrn0qsS2’.
    It is a member of the hidden package ‘vector-0.10.12.3@vecto_1COyUuV1LrA1IjYnWfJnbs’.
Prelude> 
Leaving GHCi.
% ghc-pkg-7.10.2 list vector | cat
/usr/local/lib/ghc-7.10.2/package.conf.d:
    (no packages)
/home/dmwit/.ghc/x86_64-linux-7.10.2/package.conf.d:
    vector-0.10.12.3
    (vector-0.11.0.0)

% ghc-pkg-7.10.2 describe vector-0.10.12.3 | grep 'key:\|exposed:'
key: vecto_1COyUuV1LrA1IjYnWfJnbs
exposed: True

I have tried reproducing this with other packages in a handful of ways and failed; so I can't give instructions for reproducing. But I am happy to perform any diagnostics you can think of.