I would calculate the Cap Rate for both situations. The current one and what the property would produce if fully occupied minus a reasonable vacancy factor with increased rents. But you better be darn sure that you can obtain the rent increases and it’s not pie in the sky thinking.
One of the key decisions in purchasing a multi-family apt complex is a CAP Rate. My question is: What figure do I use in calculating the cap rate (the actual or full occupancy)? And the offering price should be based upon past and existing occupancy or its potential occupancy?