1. An implantable cardiac stimulus device configured for use with an implantable lead having a plurality of electrodes, the implantable cardiac stimulus device including operational circuitry having sensing inputs for coupling to at least three electrodes such that at least first and second sensing vectors are defined, the implantable cardiac stimulus device being configured to perform a sensing vector configuration sequence in which first and second sensing vectors are selected as primary and secondary sensing vectors, respectively, and the operational circuitry is configured to perform the following while sensing cardiac signals for the purpose of determining whether therapy should be administered: maintain an X/Y counter in which X represents a quantity of recently detected cardiac cycles in which an arrhythmic condition is likely and Y represents a total number of recently detected cardiac cycles, while using the primary sensing vector to identify and analyze cardiac cycles for purposes of maintaining the X/Y counter; and upon reaching a first threshold for X, and before reaching a second threshold for X, analyze at least one cardiac cycle using the secondary sensing vector to confirm whether an arrhythmic condition is likely.