Fuel is the fed to the engine only with the engine driven fuel pump, which should be enough.
Not quite, because of this:
...and to prevent vapor lock in the fuel lines at high altitude.
Drop tanks were frequently used at high altitude and if they ran on the engine pump alone they may have run into vapor lock issues (since the engine is pulling
fuel from the tanks rather than being force-fed), however the external tanks do have a boost pump of sorts, it's just pneumatic rather than electric and there are no pilot controls for its operation. The vacuum pump pulls air through all the gyro instruments to spin them but after it goes through the pump that air (which is now under pressure) has to go somewhere. In most planes it's just dumped overboard but the P-51 puts that "free" air pressure to use by routing it to the tops of the drop tanks which forces fuel through the pickups at the bottom. So basically your suction gauge is telling you both the inches of vacuum at the gyros and the inches of pressure in the drop tanks (2" Hg = about 1 psi), assuming they're installed and there are no leaks. That same vacuum pump also provided pressure for the pilot's G-suit. Pretty brilliant concept for what was once a single purpose piece of equipment.