Thanks for replies so far, folks.
@David: My Vitessa (not an L) has got a slit in the axle of the take-up spool, not a clip on the spool. It is seemingly without any tooth or other device to hook a sprocket hole. But I'll look again tonight. On some loading attempts, I did try to start winding on before closing the back, as you suggest. The sprockets did not engage the sprocket holes unless I pressed the film onto the sprockets with a finger; then the film would go taut and wind; but as soon as I let go and re-mounted the back, the sprockets would stop engaging, and the film wound no further. Hence my trying a second and different film, just in case the first one was outrageously curly (curling away from the sprockets), though I didn't think it was.
@jhw: Good to know the way it opens is seemingly OK. I had started trying to catch/slow the doors and front standard like that, at which point it became clear to me that I need to master the art of doing so without getting a palm-print on the Ultron!