Positive thoughts and hugs from here
My first Cocker became ill with the same symptoms, very suddenly, when he was 15 years old. It was a liver infection of some sort and he had a very rough couple of days where he mainly slept, was very lethargic and floppy and couldn't keep anything down. I was almost at the point of letting him go when he suddenly perked up. The infection and it's cause was never fully identified, but old Josh lived another 2 years without any recurrence or any long term problems caused by the infection.
I hope Vegas bounces back just as well as Josh did.