As long as the cable is tight enough for the working range of the lever to cover the working of the derailleur, cable tension doesn't matter in friction (non-index) shifting. You're adjusting the trim yourself after each shift so there's fixed relationship between the levers and RD position to adjust.
As far as the vibration you're feeling, if you can't resolve it by trimming the RD, it's probably related to sprocket wear. Since you don't ride on all your sprockets equally, some wear more than others. As the chain stretches it is better mated to the sprockets you ride the most, and will run less smoothly on the ones you ride the least.
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