What is the slang word for a vibraphone?

There isn't really a widely used slang word for a vibraphone. It's often just called a "vibes" or "vibraphone" in casual conversation.

Some musicians might use terms like "vibes machine" or "vibes rack" to refer to the instrument, but these aren't common slang terms.

The vibraphone is a rather specific instrument, so there hasn't been a need for a lot of slang terms to describe it.