I've got the answer (I have the green tick) and I know I'm being pedantic but where is the instruction to replace one letter with another? After the "not very" it just says "quiet" - if it said "not very but quiet" then it could inferred that 'quiet' replaces 'very', but, of course, it wouldn't make any grammatical sense. As it stands, it's sort of in a no-man's land - it makes sense grammatically but you're left to your own devices as to what you do with "quiet".
I think we're just reading it differently Geeker, but that's part of the beauty of cryptics 😀