1. As mentioned above, yes you can use effectslab with any NLE but you can only import .MOV and .AVI video into effectslab.
2. Vissionlab is both compositelab and effectslab in one application with a few extra features (such as the lightning engine) and a full commercial licence frown in. It is a lot faster and more practical to be able to do compositing and visual effects in one application instead of switching between the two; also a commercial licence is vital if you want to charge for your work ECT.
3. The main reason to go with Vissionlab is that you can do a lot of cool effects that you can do in AfterEffects but for a fraction of the price. But if you are interested in some slightly more advanced visual effects and want to go in to the visual effects business it might be better to buy AfterEffects.
Hope this helps