Hi Linda,
Welcome to the Q&A Board!
I have fz3 and, thus far, have found little if any noticable vignetting. I personally have not used a conversion lens but I've heard numerous people recommend the Olympus WCON-0.7x Wide Angle Conversion Lens. You can get a description of it here:
http://www.adorama.com/IOMWCON07.html?searchinfo=olympus%20wcon-07&item_no=3Make sure when you purchase the camera and lens that you will be able to exchange it or get a refund if it doesn't meet your expections. You may pay a little more by going to a local camera shop, but many other places charge a hefty restocking fee (abut 15%). A local camera store can also advise if you will need a step-up ring for the converter lens.
The fz3 does a fairly good job in low light, if there is contrast in an image and you use Image Stabilization. However, based on my I would not call the low-light capabilities excellent. It's on par with other digital cameras in it's class. One nice thing is that you can switch a AF assist lamp to pre-focus, so you have the benefit of the focus assist without using a flash.
I really love my fz3.