Evidently, instead of

vrfs_in = OpenStudio::Ruleset::makeChoiceArgumentOfWorkspaceObjects("vrfs_in","OS_ZoneHVAC_TerminalUnit_VariableRefrigerantFlow".to_IddObjectType,model,true)

something like

vrfs_in = model.getObjectsByType("OS_AirConditioner_VariableRefrigerantFlow".to_IddObjectType)

seems to generate an ".eachable" Ruby collection.