This ice cream tastes very good.
✗ This ice cream tastes vary good.
Ice cream brands vary in quality.
✗ Ice cream brands very in quality.
"Very" is used for emphasis. Calling something "very good" implies it's better than something that's merely "good".
This ice cream is very sweet.
The ice cream is unusually sweet. All ice cream is sweet, but this ice cream is very sweet.
"Vary" is a verb meaning "to differ" or "to change".
I vary what flavor of ice cream I eat.
This person doesn't get the same flavor every time.
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