Jason,
I don't buy that detailed analysis. Local vacancy rates are irrelevant for a single house. If you keep it rented the entire year, your rate is zero. If it goes three months for a fixup and re-rent, you're at 25%. Either could happen. A much more relevant number is how long it takes to fill a vacancy.
Some expenses are fairly stabil. Taxes, insurance, utilities the owner pays. Others are highly variable - major tenant damage, evictions. Others are irregular, but frequent - routine maintenance, snow removal, mowing.
If you're looking at a 24 unit apartment building, 50% of rent is a pretty widely used number. That includes vacancies, expense, and capital reserves. For a single SFR (or duplex) rental, its more variable. In a good year, it might be 25-30%. In a bad year, it could be 150%.
Don't try to do too detailed of an analysis, and convince youself - this won't happen and that won't happen, therefore my expenses will be really low.
Financing doesn't really change what's a good deal and what's a bad deal. Financing is dependent on your personal situation. I usually look at deals two ways. Either assume 100% financing. If the property works with 100% financing, it will work better with a down payment. If it doesn't work with a 100% assumption, throwing in a big down payment won't make it a good deal. Or, figure the NOI divided by cash invested to get cash-on-cash. If that's at least 15% with realistic expense projections, it meets my cut-off.
Jon