Read the vacancy clause before you read the premium. The exclusion that empties investors' pockets is water. A pipe splits in a freeze, nobody's there for six weeks, and you find out the vacant policy you bought excluded freeze loss unless heat was maintained or the system was drained and shut off at the main. That condition is standard language in a lot of vacant forms and it's a condition you have to actually perform, not just agree to.
Which runs straight into the other REO problem. The servicer winterized the house, so the water is off and antifreeze is in the traps. You cannot test the plumbing during your inspection period, and you often cannot turn the water on without the utility and the seller agreeing. So you're insuring against a failure mode you also can't inspect for. Price a full repipe as a possibility in the acquisition number on any house that has sat through a winter in a cold state.
On premium, treat ledger's two to three times as a starting range and expect a per-carrier inspection requirement within the first 30 days, plus a higher deductible than you're used to. Some vacant forms are actual cash value on the dwelling rather than replacement cost, which matters enormously if the house burns before you finish. Ask for that specific term in writing.
The licensing question also has a second edge nobody raised. If you plan to bid at auction for other people's money, that starts touching securities rules depending on how the money is structured, and that's a conversation for a securities attorney before you take the first dollar.