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In Harry Potter and The Deathly Hallows, while in Gringotts when Harry, Ron, Hermione and Griphook are disguised, they pass through the thief's downfall and just fall toward their destination. Wouldn't the thief's downfall be placed somewhere where people could not just fall out right next to their destination, which is claimed to be one of the most secure vault in Gringotts?

In the novel (and film), Hermione casts a spell to prevent them from ker-splatting.

They were deeper than Harry had ever penetrated within Gringotts; they took a hairpin bend at speed and saw ahead of them, with seconds to spare, a waterfall pounding over the track. Harry heard Griphook shout, ‘No!’ but there was no braking: they zoomed through it. Water filled Harry’s eyes and mouth: he could not see or breathe: then, with an awful lurch, the cart flipped over and they were all thrown out of it. Harry heard the cart smash into pieces against the passage wall, heard Hermione shriek something and felt himself glide back towards the ground as though weightless, landing painlessly on the rocky passage floor.

‘C – Cushioning Charm,’ Hermione spluttered, as Ron pulled her to her feet: but to Harry’s horror he saw that she was no longer Bellatrix; instead she stood there in overlarge robes, sopping wet and completely herself; Ron was red-haired and beardless again. They were realising it as they looked at each other, feeling their own faces.

As to why they ended up closer to their target, the simplest answer is that they were already spiralling down (on a rail track) to get to the bottom of a deep well and access the deepest vaults. Falling just got them there quicker.