Hi flyingmoose, that is some good advice that @waylow gave you. That way, you will have the most control over the asymmetry.
Unfortunately, there is no way, that I know of, to assign a Vertex Group to the Mirror Modifier.
You could make part of the bracket (some vertices) a separate object,(without a Mirror…), but that would be a bit of a ‘clumsy’ way of working, I think.
You can also use any Deform Modifier after the Mirror Modifier to destroy the symmetry undestructively; for instance a Lattice:

But in this case, with the lock bracket, I would apply the Mirror Modifier, like Wayne said.