A vision system for a vehicle includes a camera having an image sensor and a lens, with the lens exposed to the environment exterior the vehicle. An image processor is operable to process multiple frames of image data captured by the camera and processes captured image data to detect a blob in a frame of captured image data. Responsive to processing a first frame of captured image data, and responsive to the image processor determining a first threshold likelihood that a detected blob is indicative of a contaminant, the image processor adjusts processing when processing subsequent frames of captured image data. Responsive to the image processor determining a second threshold likelihood that the detected blob is indicative of a contaminant when processing subsequent frames of captured image data, the image processor determines that the detected blob is representative of a contaminant at the lens of the camera.