Nothing I can say is official. The official answer is already presented to you in the books. (Yeah, I know, that doesn't help cos Mongoose's books have way too much room for interpretation which are easily seen as errors).
Let's look at the real world. The Bradley IFV is equipped with a 25mm Bushmaster M242 auto cannon.
Wiki says that:
"The M242 has a single barrel with an integrated dual-feed mechanism and remote feed selection.[1] The gun has 300 rounds of ammunition in two ready boxes (one of 70 rounds, the other of 230 rounds), with an extra 600 rounds in storage (in the M2 Infantry Fighting Vehicle variant) or 1200 stowed rounds (in the M3 Cavalry Fighting Vehicle variant)."
OK, same cannon mounted in a different US vehicle, the
LAV-25. Again according to Wiki:
"armed with an M242 25 mm chain gun with 420 rounds of 25 mm ammunition, both M791 APDS-T (Armour Piercing Discarding Sabot-Tracer) and M792 HEI-T (High Explosive Incendiary-Tracer), of which half is ready for use. One hundred fifty rounds are ready for use from one stowage bin, 60 from another stowage bin, the other 210 rounds are stowed elsewhere in the vehicle."
The same cannon (or at least a version of it) is carried by the
Ticonderoga class cruiser, Wiki doesn't list the amount but somehow I think it will be more than the Bradley!
Coming back to the game, as you can see in Real Life, there is no set size for a magazine for this weapon. It can be whatever suits the vehicle it's mounted to. The cost of the metal can that the ammo is in is small and can easily be included as part of the weapon price.
Q. How much does the magazine cost?
A. It's included in the cost of the weapon.
Q. How much does the ammo cost?
A. Depends on the type and quantity. The weapon (in it's vehicle) is shipped without ammo.
Q. How much ammo is included with vehicle X?
A. As much as will fit in the vehicle. I am not yet familiar enough with the new VHB to answer this. If as I suspect the rules are vague I would pick a number and call it official. For vehicles the size of the LAV or Bradley lets say 500 rounds. If the vehicle has cargo you can carry more ammo.
Lastly, I draw your attention to the following on page 6 of the VHB:
"Referee Fiat
While you can go a long, long way designing vehicles with the rules presented in this book, creating everything from unicycles to advanced hypersonic airliners capable of traversing alien atmospheres, the Vehicle Handbook is not intended as a hard set of mechanics. Rather, it is a toolkit to provide referees and Travellers with a framework with which to create their designs, rather than limit them.
The referee, therefore, is always free to ‘jiggle’ things and smooth out any rough edges in order to create
the vehicles he wants to see in his setting. We have intentionally put a lot of ‘fuzziness’ into the design system (such as with the space allocated to passengers and the Speed Bands) to make this easier but if the referee wishes to go further... he has our blessing!
If, for example, you want to create a helicopter that has a special fuselage that is shaped to float on water, don’t feel you need to add the Floats option. You are free to say that ability is part of the vehicle’s natural design. If you want to build a large vehicle that can nonetheless fit inside the cargo hold of a Subsidised Merchant, again, simply tweak the Shipping Size of your final design.
The Golden Rule is that the final designs must fit into the universe you have created. It is perfectly acceptable to adjust any characteristics of a vehicle in order to do so."
The official answer is that you get the final say on what makes your vehicle tick. "Pick a number between X and Y"