DIAGNOSIS This is insulation on motor windings or wiring overheating. It's a serious warning sign. The most common cause is dust/debris inside the motor housing insulating it and trapping heat.
FIX Stop using it immediately. Unplug it. This requires internal inspection. Remove the base plate. Look for dust bunnies, especially around the motor and the fan on the armature shaft. Vacuum it out carefully. Look for discolored (browned or blackened) wires or insulation.
IF THAT DIDN'T WORK The motor itself is overheating due to worn bearings or shorted windings. Do not continue to operate it. This is a job for a professional technician or Vitamix service.
HEADS UP A burning smell is not normal. Ignoring it can lead to complete motor failure, melted wiring, or in extreme cases, an electrical fire. Do not assume it will "burn off."