Triangular route.

Let say I am a Japanese airline and I want to fly to HRB and VVO. If I launch a route like KIJ-HRB-VVO-KIJ, while I obviously won't have the passenger transfer right between HRB and VVO, can my KIJ-VVO and HRB-KIJ passengers use the segment of the flight and potentially fill that segment of the flight full?

Yes, but the flight will be even more disadvantaged than usual. Passengers will fly up to a maximum of three legs to get to their destination, and the triangle counts as two of those segments.