A model test of a free-hanging riser under vessel motion and uniform current is performed in the ocean basin at Shanghai Jiao Tong University to address four topics: 1) confirm whether vortex-induced vibration (VIV) can happen due to pure vessel motion; 2) to investigate the equivalent current velocity and Keulegan-Carpenter (KC) number effect on the VIV responses; 3) to obtain the correlations for free-hanging riser VIV under vessel motion with VIV for other compliant risers and 4) to study the similarities and differences with VIV under uniform current. Top end of the riser is forced to oscillate or move, in order to simulate vessel motion or ocean current effects. Fiber Brag Grating (FBG) strain sensors are used to measure the riser dynamic responses. Experimental results confirm that the free-hanging riser will experience significant out-of-plane VIV under vessel motion. Meanwhile, vessel motion-induced VIV responses in terms of response amplitude, response frequency and cross-section trajectories under different test cases are further discussed and compared to those under ocean uniform current. Most importantly, the correlation among VIV response frequency, vortex shedding pairs and maximum KC number KCmax is revealed. The presented work is supposed to provide useful references for gaining a better understanding on VIV of a free-hanging riser, and for the development of future prediction models.