I think 'vice versa' is used incorrectly here.
people's tendency to spend more when they get richer, and vice versa
What they've written (with 'vice versa') is: "and people tend to get richer when they spend more".
What they probably mean is: "and people tend to spend less when they are poor".
You can remove a parenthetical phrase/clause, and still get a complete sentence:
Given the well-known wealth effect, that alone would translate into an expected drop in personal spending of between five hundred and seven hundred billion dollars.
That alone (the wealth effect alone) would translate into an expected drop ...
Basically it means "People are now poorer, and that would mean a drop in personal spending".
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