Hi Greg - From a lender's standpoint, the main difference between a vacation home and investment property is how you plan to pay for it. If you can qualify for the loan based on your current income, go ahead and try to buy it as a second home. But if you need the rental income to qualify for the loan, the lender will insist on treating it as an investment.
One other key point - a vacation home must be at least 50 miles from your primary residence to qualify as a second home. Otherwise, lenders will treat it as an investment property. Investment properties are charged a higher mortgage rate than vacation homes, and both are charged higher rates than you'd pay on a primary residence.