I was watching the scene in Deathly Hallows where Griphook takes Harry et al into the Lestranges' vault at Gringotts so Harry can retrieve the Hufflepuff cup Horcrux, and I got to wondering how Snape managed to get into Gringotts while carrying a weapon (it was a replica of the Sword of Gryffindor, I know, but it was nearly flawless) and get all the way down to the Lestranges' vault without assistance. Even taking the Imperius Curse into consideration, how did Snape know where to go or how to operate the Gringotts carts that access the vaults? Has J.K. Rowling addressed this anywhere, or is it mentioned in canon? Obviously, the Lestranges wouldn't have assisted Snape in swapping out the two swords. Is it likely Snape would have been able to pull the caper off alone?
Snape didn't place the sword into the vault himself; he handed the (fake) sword to Bellatrix (presumably, or to her husband, or to her brother-in-law, it makes no difference), and they went to Gringotts and ordered it to be put in their vault — which was personally done by Griphook, in the one single time he went to the Lestrange's vault.
None of this information is addressed in the movies, but it is fully explained in the books, firstly in chapter 15 (which has Griphook talking about in in the woods) and then later in chapter 25:
‘I have visited the Lestranges’ vault only once,’ Griphook told them, ‘on the occasion I was told to place inside it the false sword. It is one of the most ancient chambers. The oldest wizarding families store their treasures at the deepest level, where the vaults are largest and best protected …’
Therefore the real sword was never in the vault (except when Harry took it; and he didn't have much trouble in doing so, although it's hard to say with all the mayhem) and Snape never had to retrieve it; he made the exchange before giving the sword to the Lestranges.
(I know you already found the answer, but since there are a number of people that didn't read the books and may be confused, I thought it was better to answer it anyway)
Although we learn in chapter 15 of Deathly Hallows that Snape sent the fake sword to Gringotts, it's not stated how this was accomplished. (See p. 289 (Arthur Levine 13th ed. hardback.) The information is conveyed through an conversation among Griphook and Hogwarts classmates in the forest, overheard by in-hiding Harry, Hermione, and Ron.) Generally, it seems that because any item (such as a sorcerer's stone or cup) can be carried into Gringotts to be placed in a vault, Bellatrix could very well have carried the sword herself into her vault (although this is merely a reader's speculation).