Plastic welder: My new favorite tool.
So
how do you get the grain into it from the vinyl? Heat it and press on to use like stencil?
For my trim repair, I was impressing the grain using a pattern from the vinyl repair kit into the soft spot putty before it hardened, then touched up and smoothed with a wet finger manually. I've had my kit for decades, and used it for vinyl repair in the past which was marginal. It looks like they are still made. For this trim repair I only used the grain pattern which is pressed on the soft material like a stencil.
The original intent of that vinyl repair kit was to use a liquid vinyl-like compound and impress in it with a pattern then let it harden. The patterns are a reverse grain. I've done similar using a like grain patch of vinyl, but because it is not a reverse pattern, it's not exact, but workable in a not so visible spot.
Heating the plastic and trying to impress in the molten or soft plastic would not work unless the reverse pattern or vinyl patch could withstand the heat, such as metal, and release from the soft plastic without sticking, perhaps using something as a release compound.
The closest I've done directly on plastic that was noticeably visible by melting with heat was a cigarette burn in an E-body console lid. Although the tool in the aforementioned kit would work for melting the plastic, in this case I used a soldering iron to melt and smooth the plastic, then hand formed the grain with a dental pick while it was still soft, followed by light sanding and hand buffing. It was not an invisible repair, but it wasn't noticeable with a casual glance.