What substitutes for prayers for the Pope's intentions during sede vacante? One of the requirements one must meet to obtain an indulgence is to pray for the intentions of the pope. When there's no pope, what's the substitute? I guess it's the conclave and the next pope, but I don't know the exact rules on this.
It's not a prayer for the Pope; it's a prayer for the intention of the Pope.

From the Enchiridion of Indulgences issued on 29 June 1968:


  26. To acquire a plenary indulgence it is necessary to perform the work to which the indulgence is attached and to fulfill the following three conditions:
  
  &bull; sacramental confession,
  &bull; Eucharistic Communion,
  &bull; and prayer for the
     intention of the Sovereign Pontiff.
  
  It is further required that all attachment to sin, even venial sin, be absent.
  
  If the latter disposition is in any way less than perfect or if the prescribed three conditions are not fulfilled, the indulgence will be partial only.
  
  29. The condition of praying for the intention of the Sovereign Pontiff is fully satisfied by reciting one Our Father and one Hail Mary; nevertheless, each one is free to recite any other prayer according to his piety and devotion.


Although certain functions of the Papacy are carried out during a vacancy under the Cardinal Camerlengo, there is no "Sovereign Pontiff".

Given that a prayer for the intention of the Sovereign Pontiff cannot be properly fulfilled in a vacancy, it would appear that any indulgence will be partial only.