I don't think your bag is a true vachetta bag. What makes you think it is? Vachetta is a very light (almost white when new) leather that is totally untreated and has no color added. As it's exposed to sunlight it darkens naturally.
If you're familiar with Louis Vuitton bags that have vachetta on them then you'll know what I mean.
Your bag looks like it has some color treatment, not vachetta. I think your bag has been 'burnished' and somewhere there is a guide from
Coach how to treat each of their types of leathers.......maybe someone more experienced can chime in here to help?
*Sorry, I meant to respond directly to krtFL*