Disclaimer: Bryss' Improvised Points Theory is improvised. The costs I lay down are costs I think are close to the GW method whilst subtly improving it. It is not a definite theory by any means.
Vehicles are integral to 40k. From the plastic bricks you put your men in to the plastic bricks you shoot with, the vehicles of this game are many and varied. Sort of. To be honest, you can't do nearly as much with vehicles as you can with infantry, due to there being less variables involved overall. I suspect it may be hard to cover everything, but I will try my best, using my regular format. These hints mainly apply to designing a new race (like the Troops one was), but some of the principles could be applied to making one for an existing race.
1. Aesthetics: There are 125 combinations of vehicle armour (assuming limits of 10-14). Of these, any where the rear armour is better can be struck off, because a player could turn a 10-12-14 vehicle 180 degrees and use it like a 14-12-10. This is much less than the various infantry combinations and so when making a vehicle unique you have to consider what it looks like much more.
Example: If you make a box shaped vehicle that is 11-11-11, you've made a slightly better Rhino. If you make a more triangular vehicle that is 11-11-11, you have a Fire Arrow APC.
You'll notice I didn't say to always make the Front Armour better. This is because I think a case could be made for having better Side armour, and because you shouldn't always follow the GW norm.
Example: You decide your army should be mounted in futuristic airships with all the cannons located on the vehicle sides. Since this is the side you would typically present to your enemy, why shouldn't you have better armour on the sides?
Finding vehicle design inspirations isn't hard. A good place to look is finding clips of those old shows where people built robots with weapons and had them chop each other up (I have fond memories of watching Robot Wars as a child). Of course, your vehicles might be skimmers, or not armed with a hydraulic flipper, so designs can be modified as you see fit.
A final note on skimmers with wings. Don't worry too much about whether or not it would fly in real life unless you want to. 40k aircraft can be downright weird.
2: Typing: When you consider looks, you'll usually consider this as well. Vehicle classes can be split into two categories; General and Specialised. General categories are more about the base design, and Specialised categories put your vehicle into more of a niche, like so:
Examples: Tank and Skimmer both affect the looks of the vehicle and add general movement rules, so are General categories. Fast and Open-Topped are Specialised, as they turn a vehicle into something capable of outflanking quickly or a cheaper, fragile vehicle. Transport is of course the most Specialised of them all, adding an whole new ability to the vehicle. Walker is a special case as it is both.
Most of these things have been fused together at some point, however there are a few that are untouched. Of course, there is no requirement that your vehicle HAS to have any of these attributes at all.
Example: Look at the Ork Trukk. It is neither a Tank nor a Skimmer. If you removed the Open Top, Fast and Transport rules you still have a perfectly functional vehicle.
The typing should affect the cost of the vehicle, but not as much as the weapons, rules and armour facing. Using Bryss' Improvised Points Theory, I class all of them, excluding Walker, as 5 points each, except Fast, which is 10 if given to a Skimmer, and Transport, which gets more expensive the tougher the vehicle is.
Example: Using the 35 point Trukk as a base, it rises to 45 points if it was a Fast Skimmer, 40 if it was a Tank and a miniscule 20 if it had nothing but its Big Shoota.
3. Armour : This shouldn't take long. The armour values and BS make up the base points cost of the vehicle (if it isn't a Walker), to which you can add guns, class and special rules. The basic vehicle hull goes up to about 150 for 14-14-14, and goes down to 15 for 10-10-10 under BIPT. The midpoint of 12-12-12 is 75.
3.25: Guns: Gun costs are added at this same step, which is why this isn't point 4. There generally is no difference in points cost between infantry and vehicles so using wargear costs works well, sometimes at -5 to the original cost. If your vehicle spawns with a defensive weapon (or the equivalent of a Heavy Bolter), cost that as 5-10.
3.5: Special Rules: Like with Monsters, I cost them as I do for characters, usually between 10-20 depending on how beneficial they are.
3.75: The little things: Things like smoke launchers and searchlights should be assumed to be free in the same way as universal army special rules. Fire Points should be costed according to their number, my general caveat is 5 for 2 and 10 for 5. Any more than 5 and you should probably consider being Open-Topped instead unless you want to be more expensive and have the protection.
Example 1: The Raider. If we assume 15 for 10-10-10, add 15 for Fast and Skimmer and add the cost of a Dark Lance for Warriors, we get 55. Add 5 for capacity and we're there at the actual cost of 60. Note I haven't costed Open Top here. This is because at this level it greatly weakens the vehicle as well as strengthens it.
Example 2: The Rhino. If we assume 20 for 11-11-10, add 5 for the Storm Bolter, 5 for Tank, 5 for capacity and 5 for Fire Points, we get 40. Because I think the Rhino is underpriced, my theory still works.
Example 3: The Land Raider. If we assume 150 for 14-14-14, add 10 for the Heavy Bolter, 60 for the lascannons, 20 for Transport (including Assault Vehicle) and 10 for Power of the Machine Spirit, we get 250.
4: Ordnance: Ordnance weapons are unique to vehicles, and as such tend to be ludicrously expensive. They should really be S6, AP4 or better and cost no less than 20 points with no blast, 25 with small blast and 30 with large blast. Avoid going over 60, as that sort of cost is getting into the range of the overcosted already unless there is a good reason for it, or it is mounted on something with good armour.
Good Example: The Demolisher Cannon. Mounted on a fairly tough 115 point vehicle, its cost is made much lower by its short range, to about 65 mounted on a 50 point platform.
Bad Example: The Killkannon. Mounted on a fairly tough-ish 90 point platform, its cost is 5 less than my assumed Demolisher cost despite being weaker and having the same range.
5: Walkers: Walkers are strange to cost, and I can't even split them between those with Dreadnought weapons and those that don't, because there are too many exceptions. I can't use the vehicle version of BIPT on them, so have some general pointers:
The armour and free extras parts of BIPT still work, but that's it. Because Walkers have many of the stats of infantry, which are not costed per stat, their final cost is a lot more subjective. Use your gut.
When costing Dreadnought Close Combat Weapons, it should fall between 15 and 25. Bizarrely, GW seems to cost them based on Strength like with infantry even though the modified S is still 10. I would personally cost them based on WS and Attacks instead, as it is a fairer method.
Example: The Killa Kan is 55 with a DCCW and Kustom Mega-Blasta, and the Dreadnought 105. If we improve the Kan's armour, WS, BS and I to Dreadnough levels, I get to about 90, 95 if I make the KMB AP1. That's a 10 point difference between the two without changing Strength, so I have a feeling I may be right about the DCCW costing.
Gun prices should take about a 10 point drop if they replace a DCCW included in the base weapons of the Walker. If they replace a gun, the drop should only be 5-ish. Don't be scared to use numbers that are not a multiple of 5 if you feel it is right.
That's all for now. Like I did for Troops, I'll give an example vehicle from the very tentative Disciples of Sky experiment soon.