Vanishingly means so small that it is almost nothing at all — small enough to be near zero. It is an adverb and it does not stand alone; it goes in front of an adjective, and in practice almost always in front of small, rare, thin, few, or unlikely. ‘Vanishingly small’ is by far the most common pairing. The picture inside the word is a thing shrinking until it disappears, and that is exactly the feeling: not just small, but on its way to nothing. It belongs to careful, educated writing — science, statistics, journalism, opinion pieces — and would sound odd in casual chat, where you would say ‘almost zero’ or ‘basically never’. It is stronger than very and much stronger than quite. Writers reach for it when they want to say ‘yes, technically possible, but stop worrying about it’.

微乎其微地、趨近於零地（幾乎小到不存在）