Mike,
I think you have the most likly answer, I also experienced this issue yesterday afternoon for the very first time. I had everything set up correctly in the FMC and the VREF speeds would not transfer to the tape no matter how many times I clicked them. Just to satisfy my own curiosity I departed, since I knew what the references already were, and found that VNAV would not intate either. I closed everything out and re-started and flew 3 more consecutive flights without the error re-occuring. I use my preferred panel state for each flight and think that something just hiccuped in this particular case and a reset did the trick for me.
Cheers,
Jeff