In the lending world the primary difference between a vacation home and an investment home is the amount of equity you are required to bring to the table at closing, 2^nd home/Vacation homes allow for a 10% down payment while an investment home requires a minimum of 15% down payment. 

Also, an investment home purchase allows you to use the projected rental income as income for your loan in order to lessen the impact of the new mortgage on your DTI (Debt to Income) while 2^nd homes/vacation homes require you to afford the payment without the use of any rental income.