I think I have this totally figured out. Vibraphonist Carolyn Stallard came over and we experimented with the Late Night Mallets (click here for the mallets).
We played on my first floor and went and took turns listening in the basement. If this was an apartment it would be a poorly designed apartment. I imagine most apartments have better insulation than the floor to my basement.
First the Late Night Mallets really cut the sound down. You can really hit the instrument and work on your hands without producing a lot of sound and bothering someone. No more playing with soft mallets but still playing very very softly. The Late Night Mallets will produce just enough sound for you to hear the instrument and the articulation. It's totally acceptable. No one in the next room would hear the music, or at least hear a tiny tiny bit of it. So problem solved, you have a baby sleeping in the next room but you want to practice or you have your baby in a snugly and you want to practice and keep the baby asleep; You can do it.
The next problem was getting rid of the thud we were hearing in the basement. Well actually it was reducing it as much as we could. We really reduced it a lot. I put down two moving blankets and it made a difference, however I said to Carolyn we need more blankets. Carolyn being smarter than me said, 'No we don't, just fold these over. We just need them under the wheels'. So we did that and it looked like this.
Now we had the equivalent to 4 or 5 blankets underneath. The thud was reduced by about 85-90%. It was so quiet that I think most neighbors would be fine with it. With music or a tv on I think they wouldn't notice it.
We talked about a neighbor who still complained and we thought of positioning the vibes over a room that wasn't used a lot. Maybe positioning the vibes over the neighbor below's kitchen? Maybe their bathroom? Maybe their bedroom during the day? You may need to experiment here and maybe even get the neighbor involved.
We were both amazed at how the Late Night Mallets and the moving blankets made a difference. You couldn't play with regular mallets in most apartments, especially late at night. The mallets reduce the sound of the instrument so much that neighbors won't hear the vibes. And the rug takes care of the thud! That means you can practice now all night! No excuses!!!!