Lisa, as other have suggested - use a leather needle and test sample first. ONE thing you need to know for sure, if a needle goes through leather - you have a hole - generally it will be there, very few times - will you be able to get that fabric to mend itself again, without alot of help. OK - now that I have scared you - realize that it may depend on the thickness of your vinyl - whether you will have difficulty - after all - when something is thin - it isn't as hard to stitch on as if it is thicker - I have used a heavy weight tear away in my hoop - spraying with a temporary adhesive then laying the leather on the top of the hoop - you have to be careful - as you can't pin it in place or use a basting stitch - as this will leave holes in your leather - so place it carefully - stitch out your design and tear away the stabilizer - now take a picture of it and post it for other to view - then you go ahead and sew up those seat cover - as others have suggested - with a roller foot and stablizer to help stitch easier. ENJOY!
First you need a special heavy duty needle, second you need to be very careful because designs like leather may 'punch' out, meaning you'll stitch out perfectly then have a 'patch' left over If you stitch to dense it'll cut the leather, maybe an iron on stablizer would rememdy this.........
Sorry, I haven't embroidered on vinyl but have sewed both vinyl and leather goods with a leather needle and roller foot on my sewing machine. Without a roller foot, I used to place lunch wrap paper, the one without the waxed surface, on top of the article as it would help the foot glide over the vinyl better. Leather is actually easier as it is not slippery.
You can find tutorials at Emblibrary for embroidering on practically anything. I have added the link below for you.
I hope this helps, hugs n roses, Meganne
Try out a small piece of the fabric first on your machine - I did a barbecue cover for my son out of vinyl and it wouldn't slide through my machine. I found by putting painters tape (the blue stuff)over what I was sewing I could get it to go through. Good Luck - what we won't do for our loved ones!
I would use a special needle for leather, but I have no experience with sewing or embroider on leather. Good luck Lisa
Can't help re: leather, but just wondering if you are talking about the original 74 seats or just covers?