Velvia... ISO 50 in 120, right? Try f;8 and 1/2 or 1 second exposures. If you want just the burst, 1/8 to 1/4 or even shorter, but it can be hard to time just right. The exposure will be mainly determined by the aperture since the fireworks are moving, the shutter speed will determine how much "action" you record. Overexposure will wash out the colors. (I'm assuming full dark.)
Experiment and take notes.