1. An implantable medical device for use in delivering therapy to a patient's heart comprising: a sensing module configured to monitor an intracardiac impedance between at least two electrodes to provide an intracardiac impedance signal and to monitor heart sounds of a patient; a therapy delivery module configured to deliver cardiac resynchronization therapy to the patient's heart; and a control module coupled to the sensing module and the therapy delivery module and configured to: acquire selection data relating to each of a plurality of paced conduction delays for at least one cardiac cycle, wherein acquiring the selection data at each paced conduction delay comprises: sensing heart sounds defining temporal fiducial points associated with at least a diastolic portion of the at least one cardiac cycle for each of a plurality of electrode vector configurations, acquiring, at each of the plurality of electrode vector configurations and about simultaneously with the sensed heart sounds, an intracardiac impedance signal, extracting measurements from the intracardiac impedance signal for each of the plurality of electrode vector configurations based at least in part on the defined temporal fiducial points associated with the diastolic portion of the at least one cardiac cycle, and determining selection data for each of the plurality of electrode vector configurations based on the extracted measurements; and use the acquired selection data relating to each of the plurality of conduction delays to set one conduction delay of the plurality of conduction delays for delivery of cardiac resynchronization therapy to the patient.