Part of my reluctance comes from one attempt at painting vinyl many years ago. Had white vinyl seats in a car and I painted them black with "vinyl paint." Didn't take long to start cracking, flaking, etc so that gave me a very jaundiced view of painting anything flexible. Admittedly that part of the Honda seat does not really flex much and I ASSume (hope!) that "vinyl paint" is better today than back then...so I ordered the stencil.
Related, and FWIW to anyone interested, in my checking around, I found that a P&K seat cover for the CB400 four is £55 + £4 shipping within in the UK and, per P&K, the Honda logo is heat-stamped as the original was. I have never fitted a motorcycle seat cover so don't know how hard/easy that might be but the price strikes me as reasonable.
I was going to do the stencil painting outdoors but decided it was safer to do it indoors for fear of Martian meteorites.