The error in the sentence is the incorrect placement of the phrase that is used to modify a noun. The phrase "with an incredible sound system" is misplaced and it should not be put after the word "vacation" because it is not "vacation" that has an incredible sound system. Rather, this phrase should be placed after the word "Mustang". So the correct sentence would be: We rented a convertible Mustang, with an incredible sound system, on our summer vacation.
