Vachetta will get darker with use and expose to air and sun even if you apply protector. It's pretty normal.
Did you use the pre treatment for new Vachetta first? Without it, new Vachetta will darken after you apply the other products. I used it on my new Vachetta first and no immediate patina. It's been months now and my purse is slightly darker, stain free, with use.
As mentioned, even with LMB the Vachetta will darken over time. You can't stop it.