with a reduced power takeoff, takeoff decision speed (lowest of refusal speed, critical engine failure speed, and rotation speed, but no less than ground minimum control speed) typically decreases in a balanced field as critical engine failure speed decreases as a function of being able to lose an engine at a lower power setting and stop or continue (accelerate stop distance decreases).
I’d also recommend FAR/AIM for a very detailed discussion of these definitions.
V1 is the maximum speed at which the rejected takeoff maneuver can be initiated and the airplane stopped within the re- maining field length under the conditions and procedures defined in the FAR’s. It is the latest point in the takeoff roll where a stop can be initiated.
Second, with respect to the “Go” criteria, VI is also the earliest point from which an engine out takeoff can be continued and the airplane attain a height of 35 feet at the end of the runway.