I have two cuttings this year (one MVSB and one Valle Calda) that rooted and began to leaf out. They continued to do fine when I potted them. But, soon after the transfer these two pots didn't get enough water and their young leaves, stems, and roots turned dry, withered and fell off. The bark of both cuttings is still green and I've put them back into slightly damp sphagnum moss, but there's no sign of growth after several weeks. I'm wondering if these cuttings should have enough energy to root and leaf out again or not.

I think we have all encountered this type of situation.  There is always hope if the bark is green but I currently am nursing along a few cuttings that have been in a holding pattern since April.  Some have green terminal buds that never popped while others are more like what you described i.e. the bud broke, a couple small leaves formed and then the leaves shriveled up and fell off.  I suspect the former are more likely to make it than the latter.  At some point they will get tossed though.  Sorry I don't have  abetter answer.

Me too.  Same situation.  I think perhaps that in my caution to not overwater them, I underwatered them.
Jim

unlikely as it has burned stored energy in rooting the first time.
you never know until you try.