Hola,
i wanted to know from you gurus when it's a good choice to use the vignetting and also when to use the Black & White.

thx a lot

Um it depends on what type of video your creating and personal choice about whether it works. Only thing I would say is that vignetting can look great but only when done well. Often you see people over use it and it becomes ugly. Take TV programs like Top Gear, they grade their videos beautifully often using vignetting. The reason why it looks so good is because they dont over do it on the effects such as vignetting and colour grading.

As for when you would use black and white and vignetting like I said its really up to you. One example of where it could be used is in a flash back scene. It also works well to create a bit more depth to a video which might otherwise look flat and often used in videos aimed more towards a younger audience, think MTV.

As with all effects/adjustments to videos subtle changes is often best.