According to these stats, a halfling or gnome could comfortably ride one of these things. 

If you use Encumbrance rules, that small humanoid would have to weigh less than 35 pounds in order to ride a vulture.  2nd level Reduce on a medium sized PC would probably do the trick. 

Vultures have a stomach acid PH-value of 1.0.
They can induce vomiting as a defence mechanism.
It would be awesome if they had an acid splash-like attack, that had 5 or 10 ft. of range.
Though it might not be a lot of damage, it is very acidic, so 1d6 or 2d4 isn't unreasonable (I think).

That would probably increase the creature's CR to 1/8 because then they could start kiting your melee attackers, and easily kill that level 1 wizard.

That sounds awesome to me.

Vultures aren't predators, so they wouldn't attack unless threatened or in some other way desperate. You would have to provoke the attack, so giving them more bite shouldn't be a problem. Perhaps the acid-ones could be a variation to show that possibility.

Vulture is classed as a Medium beast, but the Eagle is Small. Now excuse me but how in the flooding fish oil is a vulture bigger than an eagle???

Yes I know the Condor is the largest flying bird but that's one species, otherwise eagles and vultures are all roughly the same size

Bearded vultures have a wingspan of about 8.5' and weigh 15ish, with griffon vultures being roughly the same. A bald eagle has a 6' wingspan and weighs 8 lbs. 

In addition to it being medium and therefore rideable, it has a CR less than 1/4, so it is a legal option for a ranger’s beast companion. 

So, a halfling or gnome ranger could have a vulture beast companion and ride it for infinite flight starting at level 3. 

The vulture has a strength of 7, which means it can carry 35 pounds (16 Kg) before it becomes lightly encumbered; 70 pounds (32 Kg) before it becomes heavily encumbered; and a maximum of 105 pounds (48 Kg) maximum. 

A halfling or gnome has a minimum weight of 37 pounds (17 Kg), according to chapter 4 of the PHB. An exotic saddle weighs 40 pounds (18 Kg), which together make the vulture heavily encumbered even without equipment. Being heavily encumbered gives the vulture a flying speed of 30 feet (9 m) and disadvantage on attack rolls and Dexterity saving throws, but not much else that’s important. 

This doesn’t leave a lot of weight (28 pounds, or 13 Kg), but it’s still enough for a ranged weapon, some ammunition, a component pouch, a tinderbox, a mess kit, a bullseye lantern, some oil, and a few rations. If adventuring equipment isn’t carried, the vulture could be equipped with hide barding instead.

Being a gnome would give darkvision, and so avoid the weight of the lantern and oil, and casting Goodberry would avoid the weight of the rations. 

A Bag of Holding or similar item would also greatly help with weight management.

Quote from Jctvilum >>

Vultures have a stomach acid PH-value of 1.0.
They can induce vomiting as a defence mechanism.
It would be awesome if they had an acid splash-like attack, that had 5 or 10 ft. of range.
Though it might not be a lot of damage, it is very acidic, so 1d6 or 2d4 isn't unreasonable (I think).

 Wait PH 1! Like on a scale from of 14 with 7 being neutral a 1! H.F.I.L! How much by volume to they spray? Could it kill a commoner?

Eagles and Vultures have a a fair range between species, but vultures tend to be larger. Part of their feeding strategy is to “muscle out” other scavengers and this leads to them be in a bit heavier.