The reason for not steaming leather is that you can damage it. Water and leather do not mix. No leather supplier in the world recommends heating or wetting leather, and steaming does both. The reason for not steaming vinyl is that all you will do is get it wet. If the seat covers are made correctly, they will fit. If they are slightly off then adding some padding will correct them. If they are too small, nothing in the world will make them bigger.
You have to stop thinking of upholstery work as precision work, it's not. You can't adjust upholstery work by millimeters or thousandths of an inch like metal or wood. The original seat covers you copy aren't perfect, far from it. Unless they come from the rolls Royce factory, they are made in a factory on piece work, by someone who could care less about quality. The only thing they are worried about is making the most money they can. There are lots of flaws that the average person doesn't see, even in seats that look like they're perfect.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 10-05-2012 at 08:10 PM.