Would it be possible to put marimba bars on a vibraphone frame? I am trying to get a cheap marimba with a damper pedal, but cannot seem to find anything that isn't custom built by the manufacturer or built from scratch.
If the issue is to practice more quietly, there are many things you can do before frankensteining your instruments.
Adjust the vibes
On vibes, set the paddles parallel with the bars - that will cut your sound significantly and is a really easy thing to do.
Most of the marimbas I've played had springs to hold the tension on the cord. Release that somewhat and you'll soak up more sound with the cord.
Soak up the sound
I spent some time practicing with a thin case on the marimba/vibes if I felt it was too loud and ringy. It's good for playing by touch rather than sight too. You could probably also lay a couple layers of fabric or a rope on the end of the bars to soak up the resonance and get a similar effect.
If you're not practicing significant amounts of time that need to be quiet, use your fingers. This is especially useful for runs.
The next quietest is holding the sticks backwards, which is more realistic feeling but still not the right weight, hard with 4 mallets, and has an annoying tingy sound.
If you've got a pair of worn-out mallets, unwrap the yarn from them. Either use just the core or rewrap them loosely with cloth to make nice cheap practice mallets. You can also use soft rubber mallets. Just be careful about the color of the core - my favorite practice mallets sounded great but left blue marks on the keyboard.
Softer mallets can make the sound quieter as well. I use marimba mallets on xylophone and bells especially to keep myself from going deaf during practice. Bass drum beaters are very quiet on vibes, but hopefully just getting the softest marimba mallets you can find will do the trick.
Put a pedal on a marimba
If all else fails, take the kick off of a drum set and put it under your marimba. It won't do anything, but at least you can practice pedaling and playing at the same time.
Short answer: No.
Long Answer: Could it happen in this universe? The laws of physics do not prevent marimba bars from existing on a vibraphone frame. But I find it highly unlikely that you would find a set of bars that would fit. For one thing marimba bars are thicker vertically than vibraphone bars. The holes would need to line up with where the string supports are on your vibraphone and each bar would need to fit in the X&Y directions. Probably not going to happen.