You can see Lanai from Maui and it's about an hour boat ride. I would definitely take the ferry over flying, both because of the time/hassle involved with flying but also because of the expense. And the boat ride is beautiful to boot.
Not sure what you mean about vandalism/parking, but I don't see any need to take your car to Lanai. Alamo has a Lahaina Harbor drop off so when you arrive in Maui, you can pick up your car at the airport then drop it off at the harbor when you're ready to move on to Lanai. Just indicate this when you rent. Not sure of the other companies. Once you drop off your car, you can just walk onto the ferry with your luggage over to Lanai.
Once you're in Lanai, you can rent a jeep or whatever to drive around the island.
I recommend Manele Bay, which is where Trilogy takes its snorkeling passengers. The Four Seasons is on that beach and it's great for snorkeling and body surfing.
I also recommend taking a zodiak tour of the coast of Lanai. It's really a great way to enjoy the cliffs. And, of course if you're into scuba diving, some of the best spots in the world are right off Lanai -- Cathedral I and II.