What exactly do "not very good" and "not very good at all" mean in English?

Question

Here is my current understanding of these English phrases:

• very bad (=not good at all, =not very good);
• bad (=not good at all, =not very good, =not very bad);
• not good, not bad (=not good at all, =not very good, =not bad at all, =not very bad);
• good (=not very good, =not bad at all, =not very bad);
• very good (=not bad at all, =not very bad).

Is that correct?

In my native language "not very good" can rarely mean "good" but "very not good" means "very bad". So it confuses me what "not" really negates "very" or "good"?

For example, I understand the phrase "I am not very good at English" in such way that it can mean any one of these:

• I am very bad at English;
• I am bad at English;
• I am OK at English;
• I am good at English (but not very).

And I understand the phrase "I am not good at English at all" in such way that it can be any of these:

• I am very bad at English;
• I am bad at English;
• I am OK at English.

Is my understanding correct or both phrases actually mean "I am very bad at English"?

Answer 1

'Not very good' and 'Not very good at all' are intensifying descriptors of the middle, ie mediocrity. Someone who's not very good at baseball can still play the game, but they're barely competent. Not very good at all in this example would be the absolute minimum amount of skill/understanding necessary to play baseball but no more.