Can a vapor barrier follow the profile of floor joists?
No, I do not think a vapor barrier should follow the profile of ceiling joists.
Condensation occurs when vapor reaches its dew point.
Placing a portion of a vapor barrier on one side of an insulation space and not on another side (where the wood joists occur), will stop the vapor from penetrating the insulation space but not the wood joist space.
Wood is porous and vapor will penetrate the wood. As it travels through the wood (vapor moves from warm to cool) it will hit its dew point and turn into condensation... causing the wood to turn into dryrot.
If you can’t remove the ceiling material, maybe you can paint it with a good finish paint material. (Paint is a vapor barrier too.)