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I get hardly any vignetting to speak of but dust close on filters closer to the sensor is a problem because they are almost at the focal plane and so cast shadows (dust donuts) that will show up if you stretch an image enough.
The reason why you need more than 1 of each type of calibrating image is that when you take an image (even a calibrating one) there are multiple sources of noise in them - the main one being read noise from the amplifier(s). So to average out this noise you need to take multiple images and average them. Taking a single bias, single dark and single flat will likely leave your images looking like noisy trash because the read noise for each image is overwhelming any signal that would make them helpful.
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