I have been sewing quite much vinyl with my Singer 201, and the feed system is similar to the Singer 15-91.
I should look for the following:
1) Reduce thread tension (both bobbin and needle thread tension).
2) Increase needle size in order to reduce thread tension.
3) Increase presser foot down force.
This was still not sufficient for me in all cases, and then I discovered, that a changed presser foot could help you a lot. You can find information about that in this video:
https://youtu.be/rBIulDuhDDs
You may also be able to change the feed dog timing, because it can help you when sewing heavier fabrics on domestic sewing machines:
https://youtu.be/rBIulDuhDDs