The answer is you can't. Each Slide has its own unique color and characteristic. These varies again if the slide is used in different lighting condition. Velvia in the morning, Velvia in the afternoon, Velvia in overcast scene, Velvia with and without flash....All give different results using the same roll of slide.
You can try to manipulate your digicam picture in Photoshop, most likely you will get a saturated, high contrast image. However, its just a saturated color effect, IT WILL NEVER BE A VELVIA EFFECT BECAUSE IT IS NOT TAKEN WITH THE SAME FLIM PALETTE.
Having said this, you can still get a digital picture with the Velvia effect you want with this work around :bsmilie:
Take a picture of the scene using a film SLR Loaded with Velvia, then use your digicam and take a picture of the developed slide on a light panel.