OK - Your vet is an informed person on the urgency of the situation, and I'd trust his judgment that tomorrow is soon enough.
There were common sense advice tips on things to do for her, to let her be quiet, keep her movement to a minimum, and it wouldn't surprise me that she wants to lay down.
She is already in the small field, her underneath, is not quite the same as the picture, but the "tire" shape has me concerned. She has foaled before.
She is walking slowly around the field now. I am keeping as close an eye on her as i can with two elderly parents in law to nurse and a 2 year old! This is so frustrating as I lack the experience to know more.