My parents used to have one - I remember it taking ages, and needing unsalted butter (so it probably would have been easier just to buy cream).
Found these suggestions online:
To make double cream take equal amount of unsalted butter and full fat cows milk. melt in a saucepan but don't let it boil. Put in a cream maker, pump out, and leave overnight. Adjust balance for taste.
My parents used to make cream with unsalted butter, melted into milk - I think about two-thirds milk to butter, depending on thickness desired. It was pumped by hand through a nozzle with a special "cream-maker".
The best info seems to be here:
http://www.recipelink.com/msgbrd/board_27/2004 /SEP/300.html
No mention of vanilla - you'll have to experiment!