Congratulations! And I have no advice but instead empathy! I have to decide whether to fly straight to the UK for a couple of weeks and then back home for Christmas, or whether I wait four more weeks to see my fiance when he flies to be with my family for Christmas. Christmas is a kind of non-negotiable time for my family and this year they are also hoping that I'll be home for the entire month of December. I'm somewhat afraid of all the guilt and anger that would/will be directed my way if I announce that I'm flying to the UK first. And the sensible side of me sees all the $$ we've spent on visas, and even our super modest small wedding, and says "these weeks will go fast, don't pay for another flight right now."
All this to say, I understand having to make complicated decisions that your family may not love surrounding being there for big holidays and it is tough.
Unfortunately you might not have a choice in the matter. If your vignette travel dates end before Christmas, you will have to go to the UK by then. I also believe that the marriage fiancé visa is single entry (experts weigh in here?), so once you are in the UK you can’t leave until after you switch to FLR(M). You also have to be in the UK for 37 continuous days before you can get married, so if the visa is multiple entry and you can come back to the US for Christmas, keep that in mind for choosing a wedding date.
I expected that I wouldn’t be here for Christmas so my family has planned an early celebration before I leave, could that be a posibility?
Sorry to put more stress on you, just want to make sure you know the time restrictions so you don’t have more headaches down the line!