The steering rack on the variable sport steering is a different part number than the rack on the 328i without it. Also the 335i steering rack has a different part number than the 328i, although the variable sport steering is the same in both cars.
So if people are seeing vibration only in the 328i without variable sport steering, the reasonable conclusion is that it has something to do with the rack.
Note that according to Realoem.com the original part number for the 328i was 32106857283 and starting March production the part number became 32106858538. Variable Sport Steering is 32106856461