I'm going to let my girlfriend chime in on this one first. Nick's girlfriend: "Sounds like he needs to put a ring on it!" Sorry, she's been listening to a lot of Beyonce lately.
Have you thought that maybe he's planning for your future together by buying the vehicles? I mean, if one of the vehicles is a slick sports car, than perhaps not. But if he bought an SUV with plenty of room for storage and kid-friendly cupholders, then he could be preparing for your life together before getting you that ring.
(Also, I assume we're talking about an engagement ring and not just some fancy ring you saw at the store. Or the One Ring of power, forged by the Dark Lord Sauron in the fires of Mount Doom.)
Perhaps he's saving up to buy you the perfect ring. Or, yes, perhaps he's shuffling his feet. There are other ways to tell if he's getting cold feet. Does he get cagey and change the subject when you bring up wedding plans? The cars could be a stalling mechanism. Or maybe he is taking the ring for granted. He figures you aren't going anywhere, so why not buy some cars in the meantime? Maybe it's time to put the fear of God into him. I'm not saying give him an ultimatum, but don't let him get so comfortable.
Don't wait around hoping he'll buy the ring. Ask him when it's going to happen. This is the man you're ostensibly going to be spending the rest of your life with. He should be able to give you a timetable, or at least let you know what is going on with the cars. Also, from the moment he proposed, he's showing you the type of husband he's going to be. Is he going to blow money on himself instead of putting down a down payment on your future home together? Are his actions selfish, or is planning for his future with you? You have to find out if his heart is still in it, and if he still wants to get married. He could be taking you for granted, or he could be making sure you are a two-car family. But the only way you'll know is to ask him.