Best Product for Cleaning Bolsters and Seat Cushions?

Be careful. Some recommended cleaners can damage the vinyl. The Magic Eraser is one that comes to mind. If your vinyl is in pretty decent condition, avoid bleach based cleaners if possible.
Start off with a mild soap and water solution and a soft cloth or soft bristled brush. Go more aggressive from there. One product that really like is a final application 303 Protectant. It does a bit of cleaning and adds a layer of UV protection. 303 makes a multi surface cleaner also but I have not used it
 
I use Star Brite Mold and Mildew remover on the bolsters and seats. It is safe for threads. Then once clean 303 protectant.
 
I use Star Brite Mold and Mildew remover on the bolsters and seats. It is safe for threads. Then once clean 303 protectant.
Many general cleaners may contain chlorine bleach and as mentioned, that can degrade the threads. Use bleach based products as a last resort and regardless of what you use, rinse everything thoroughly with fresh water.

Also make sure you follow the directions. Some cleaners are spray on, then wipe off. Others specify an amount of time to let the cleaner sit. I have used the Starbrite product for some pretty ugly mold stains and had to let it sit. The goal is to not let it dry before wiping.