I've started accompanying myself on guitar while playing the vibe. This is a lot of fun, and solves two problems for me:
1) I'm not a very good guitarist, particularly when it comes to soloing. Plus you pretty much have to alternate chording and soloing when playing solo, which can make for a disjointed type of playing.
2) I'm not a very good vibist, particularly when it comes to chording. As a piano player, I feel limited by the fewer chord notes available on the vibe. I also have a problem with the sameness (or overload) of the vibe tones when chording and soloing at the same time.
I use one mallet and hold it between the third (middle) and fourth fingers of my right hand. This frees up the thumb for strumming the guitar strings and also allows for fingerpicking with all of my fingers except the pinky.
The trick is to keep the guitar chording going while also playing the melody and soloing on the vibe. When it's working, it sounds great, like two players in a duet. A future challenge is to play single-note lines simultaneously on guitar and vibraphone.
I've also done this by playing the chords on the piano with my left hand and soloing on the vibe with my right, using my left foot on the piano sustain pedal and my right foot on the vibe pedal. This is a little more cumbersome due to the contrasting height and width logistics of the two instruments.
I'm not aware of anyone else who's tried this. Does anyone know of any musicians who have done or are doing this? I recommend giving it a try, especially if you already play guitar or piano.