Evaluating the vehicles like they're creatures is a mistake. As @JonTheMon says, vehicles are very analogous to equipment; they're worthless without creatures (or one of the other, fringe-ier enablers), and they can be "moved around". A good first step is then comparing the statline to the crew cost.
Another major consideration is the crew cost, just on its own. A Crew 1 vehicle will almost always have a creature capable of crewing it alone. A Crew 2 vehicle is usually safe too, but Crew 3 and above gets difficult. Crewing vehicles with multiple smaller creatures is ill-advised, as now you're tapping out a lot of your board for smaller gain. The best vehicles printed in Kaladesh block are Heart of Kiran and Smuggler's Copter (the latter being strong and pervasive enough to be banned). Copter had crew 1, which made it very good. Heart of Kiran is much harder to crew, but it makes up for it by being able to use planeswalkers as crew instead.
When a vehicle has a high crew cost, you need a larger creature to use it effectively. If you have such a huge creature (for example, one with 6 power to crew Consulate Dreadnought), you are in a strong position anyway, and the 7/11 is likely not that much stronger (we call cards like this "win-more" cards, and they're generally bad). Again, maybe you crew it with a bunch of small creatures, maybe tokens, but this is bad; a major source of value from tokens is their ability to clog up the board, and tapping a bunch of them down for one large creature is very unappealing, unless that card literally wins the game.
To properly evaluate vehicles, you have to do a little more than look at their statlines. Instead consider this; when does having this card on the board, or in my hand, help me? If a card is at its most useful when you're already winning, like Consulate Dreadnought, it is probably not a good card; the Dreadnought comes down on turn 1, but you're unlikely to be able to crew it until turn 4 or 5 at the earliest, and you're either tapping down an already large creature, or giving up on your entire board to do it. It lacks trample or any evasion, so it can be stopped by 1/1s, and on top of all that it adds only 1 power on top of what damage your crew could do normally. It is at its best when the board is clogged with large creatures, so it's 11 toughness can shine. This is a very specific situation that is not even likely to come up most games. Comparatively, see limited all-star Renegade Freighter. At Crew 2, it's likely that by turn 4, when it's able to attack, you'll have one or more creatures capable of operating it on its own. Landing on turn 3 means that it arrives early enough to be relevant, but it has an aggressive enough statline to remain useful late in the game, if that comes to it. It's great at developing your board, it closes out games that you're already winning, and it is beefy enough to block for you if you're losing.
This idea of evaluating when cards are useful, instead of just looking at their stats, is a very good philosophy for evaluating magic cards in general, but it's particularly useful for weird, newer types of cards like vehicles.