Old Timers, my rosy red *ss !
As Photo Engineer and Simon Galley have pointed out, RC and FB have different, but closely matched emulsions.
Compare the published curves for Ilford FB and RC: they are close, but different. If you try to match the highlights or shadows, the midtones will not agree.
Also, the amount of development of your Fiber paper may differ from the RC paper, exaggerating the midtone differences.
This isn't unusual, and not linked to your VP (although the curve shape of VP, relative, say, to TX may point up the midtone variation between Ilford Fiber and RC )
The contact prints are probably exposed based upon an overall appearance; enlargements are probably made with more attention to the extremes of the scale, so the differences are easier to see. Diffusion enlarger, right ? So Callier isn't a problem, the contrast will be the same in the contact prints as in the enlargement.
I think everything here is as it should be.
d