Hi Haankster,
For the example that you give, the aliasing issue won't happen. When you use 400Hz ODR, the internal sampling rate is 3200Hz, 400Hz ODR is got by digital filter. So, for 550Hz vibration, 3200Hz sampling rate still can meet Nyquist's Law. Refer to ADXL345, the internal sampling rate is always 3200Hz when ODR is 3200Hz/800Hz/400Hz/200Hz/100Hz. To avoid aliasing issue, such ODR is recommended.
Hope these can help.
BR
Neil