Is it best-practice to commit the `vendor` directory?

The dep tool’s FAQ answers this:

Should I commit my vendor directory?

It’s up to you:

Pros

• It’s the only way to get truly reproducible builds, as it guards
  against upstream renames, deletes and commit history overwrites.
• You don’t need an extra dep ensure step to sync vendor/ with
  Gopkg.lock after most operations, such as go get, cloning, getting
  latest, merging, etc.

Cons

• Your repo will be bigger, potentially a lot bigger, though prune can help minimize this problem.
• PR diffs will include changes for files under vendor/ when Gopkg.lock is modified, however files in vendor/ are hidden by default on GitHub.