Do you think that the pool adds value to the unit, even considering a possibly higher vacancy rate? 

One might argue that you could be leveraging yourself up to your eyeballs, but if you know the market, the trends, the vacancy and prepare for the worst, then go for it.

If it's a buy-and-hold, will it meet the 1% rule for cash-flow (or better yet, have you broken down the numbers for debt service + taxes + insurance + PM fees + vacancy/CapEx/maintenance withholdings as compared to market rent)?

The calculation for calculating NOI for a NNN leased property is a follows:Rental Income+ Reimbursement Income (Tenant reimbursing the landlord for operating expenses)= Subtotal- Vacancy/Collection Loss (% Applied to the subtotal)= Effective Gross Income- Real estate taxes (Typically reimbursed in a NNN lease)- Insurance (Typically reimbursed in a NNN lease)- Maintenance (This will depend on if single-tenant building or multi)- Utilities (Could be nothing is single-tenant building)- Management Fees (Typically reimbursed in a NNN lease)- General/Admin Expenses (Typically reimbursed in a NNN lease)- Reserve for replacement (Tenants do not typically reimburse for this)= NOINOI/Cap Rate = Market Value

Vacancy, I use a conservative estimate of 8% or 1 month vacant out of 12.

Vacancy of 3% is your tenants staying an average of 33 1/3 months, unlikely I use 8% or 1 in 12.

It doesn’t cover basic vacancy.

Vacancy is your biggest expense as a property owner so knowing how they keep and get residents is the most important thing.

vacancy rate.

Depending on your market, you want it to be cash flow positive or at the very least, cash flow neutral (including reserves for maintenance/repairs, capex, and vacancies).