Vertex is the higher point .Vortex is lower point caused by the whirlpool.
for the 2nd meaning: more tax, is a difficult situation in which you get stuck and cannot escape.
Sounds as Vertex. A vertex of a Circle ( The round Whirlpool in this case ) Lies in the center. So Vortex is the Center of the Turbulence( disturbance ) or whirlpool. ( Imagine Circular Whirlpool ).
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