While not suggesting you do what I would do, I would say that if the Vibe part were to be played on a keyboard, I would treat the instrument as a parallel to an organist who plays a particular organ stop: I’d label the player as Keyboard and then indicate within the grand staff (and probably boxed) that the sound is to be Vibraphone. I’d then set the appropriate sound in the VST.
I do the same if I want the Keyboardist to play (or mimic) the sound of a particular synth on the keyboard.