A vacant property is going to be worth less than one that is leased up, but you can't use the cap rate formula with zero income to give you zero value.
What I do is value it leased up and then run projections on estimated cash flow for 2 scenarios
1) Your best guess as to how long it will take you to lease it up using market rents
2) Pessimistic estimates with longer time frame and below market rents ("I know we could at least fill it up at 70% of market rents for apartments)
Then we look at the cash flows over the first one to two years and use that to back into an amount you could pay and still make money
Here's what I mean by this
Normally we look at the income, estimate the expenses and use the cap rate formula to give us a value.
Then we use the Platinum Form to calculate down payment, rehab costs, and mortgage payments so we can see what the cash flow is along with the rate of return on the money invested.
With a property that needs to be leased up, you start with an estimate of the value (something less than the value if the property was leased up) enter down payment, 1st mortgage amount, and then create either year by year, or if you want more detail, month by month projections. I do this by saving multiple copies of the Platinum Form.
By doing this you'll find a value that makes sense based on the cash returns over the lease up period.