Random question but if the viginette is for a week or so after a visa approval can you enter in on a tourist visa for a week, then leave and come back when the vignette is valid?
It would be risky to try to do that, because to enter as a visitor you need to be able to show that you have no intention of living in the UK and that you have a home, a job and a life to go back to in the US.
And since you will have a settlement visa to move to the UK permanently which starts a week later, it would look odd for you to be visiting.
Why not just wait the week and use the time to enjoy your last few days in the US before you move?
Sent from my iPhone using Tapatalk