In my opinion, you shouldn't use the words for words translation for this slang. Basically, "Vãi" here is quite similar to the slang "freaking".
"Vãi chưởng" is an old slang few years ago, used to be popular for young people, meanings "freaking hell".
However, youngsters don't seem to use that much nowadays. Instead, it's minimized to become "Adjective + Vãi" to express something in a supprising way, sth extreme, or to stress the meaning of an adjective, or just a way of saying makes the person feel cool by saying it like "sick, dope, lit, etc". For example:
If you ate something and you feel really like it, you could say "Ngon vãi" (Meanings the food is freaking/ extremely/absolutely/really delicious).
If you saw a pretty girl, you could compliment by using this slang "Xinh vãi" (Meanings Yo she's so freaking pretty, but recommend to use with close friends only, some people will consider it is impolite to say this to a girl you dont know)
Etc, many other adjectives you can combine with this slang just similar to "freaking", just different order position of the adjective (Freaking + adj but with "Vãi" it would be Adj+ Vãi )