The MATLAB Support Package for Raspberry Pi includes support for real-time acquisition from Camera Board. You need to use the snapshot() methods of the cameraboard object in a loop. If you complete your image processing within frame capture time, usually 33ms, this method provides a low-latency way of capturing and analyzing images from Raspberry Pi camera. Take a look at the Green Ball Detection example included in the MATLAB Support Package for Raspberry Pi.
The command you are citing, raspivid -o vid.h264 -t 20000, just captures a 20 second video clip and saves it onto the SD card of the Raspberry Pi. You can then retrieve the video file in Simulink for online analysis. Since you cannot access the captured video while recording is in progress, I would not call this real-time acquisition.
What exactly you are trying to do with the Raspberry Pi Camera? Object tracking, face detection, etc.? I might offer better help if you detail your use case.