Only if you do a ceremony of investiture to do so. (Edit: What Alan said) And it is culturally considered as "not done". Your vassal is your servant, if he needs your help you give him GB's or support him with troops. You don't (usually anyway) willingly give away an inch of your divinely granted power.
You're obviously welcome to renegotiate any vassalage deal, f.ex. lowering the required RP payment.
I.e. RP isn't really useful as a bargaining chip.
You may, however, support his actions directly by using RP or GB in his favor.