Sorry if this doesn't help anybody, but I figured (as per the usual) that if it's something new to me, it might help someone else out a bit. So here is the mini tutorial on steaming wrinkles out of vinyl.
This tutorial is done with a garment steamer - I don't know how well this would work with an iron (if it even would) but if someone wants to give it a try with some scrap fabric, let me know the results please! Otherwise, I'll try it out some more the next time I'm working on a commission dealing with vinyl.
I'll have a couple more mini vinyl tutorials when I work on my next Shuichi commission, so hopefully that will help make a scary fabric to work with a little less scary
As per the usual, please don't say this is yours, or redistribute it without asking first. Please link people to this deviation instead