Some people swear that it needs 1-2 days of steep, some say 3-4 days.

I tend to stick with 3-4 days and it's good.

I don't get any off notes at 1% or below shake and vape. I do tend to use it in conjunction with other cream or custard flavors though and it can give a sour note for a while depending on context.

It might have been because of another flavor. Any yogurt, most cheesecakes, some custards (especially CAP), some butters, and some creams will cause that in conjunction with it. Its probably due to the cumulative concentration of butyric acid doing it. That gives a sour flavor at a certain threshold that disappears with steeping. Its a very common component in dairy flavors. Anything heavy dairy has at least a solid chance of having a sour note for a while because of this.

In high enough concentration it will actually taste very close to vomit! Should you ever want to try a new profile...

At what percent did you use it?
Steep is rather short, 1-2 days. I don‘t get any sour notes from it. Check the review and next time, please search Reddit yourself, ty.

https://www.reddit.com/r/DIY_eJuice/comments/4o6a9j/flv_cream/