It is a really nice sewing table. Sorry about the vibration problem.
Part of the problem may the material the shelf is made of. In the picture it looks like a laminated MDF (ready made shelving available at most hardware stores), which is prone to saging. Any moisture in the air will collect in the unfinished part of the shelf and change the dimensions. I suggest that you replace the shelf with solid wood, or a fully laminated shelf (also readily available at most hardware stores such as HomeDepot). If you use a laminated shelf and have to cut it, be sure to paint or seal the cut so that moisture cannot get into it. Also, use a level and made sure the shelf is level.
Good luck.