Two men escaped with a cash box after threatening G4S staff with weapons inside Santander, West Princes Street, Helensburgh, at 00:35 on Wednesday.
Police believe a dark-coloured Hi-lux 4x4 vehicle was used in the getaway.
They said a blue high-powered Audi estate and a Vauxhall Zafira may also have been used by the suspects.
The men escaped with a five-figure sum of money.
During a similar early-morning raid at the same branch on 6 April, two men escaped with another five-figure sum of cash.
Police have said the robberies may be linked.
The suspects in the latest incident have been described as being of medium height, medium build and were wearing overalls and masks.
Det Insp Douglas Wilson, from Police Scotland, said: "We continue to seek information on a dark-coloured Hi-lux 4x4 vehicle, which we understand was initially used in their getaway.
"We also continue to seek information on a blue high-powered Audi estate vehicle and also a Vauxhall Zafira vehicle which may have also been used by the suspects."
Det Insp Wilson added: "From the CCTV that was analysed, we observed passengers getting into the back of two silver private-hire taxi vehicles at around 0035 hours.
"It is possible that the taxis may have encountered the 4x4 vehicle making off from the scene at speed. If the drivers or the passengers of those vehicles have any information they believe may be useful to our inquiry - please get in contact."

Lucas Bigley, 19, stabbed Matthew Wilson to death in Sheffield on 28 May in a "violent and brutal attack".
Hull Crown Court heard the two men had fallen out after Mr Wilson, also 19, began a relationship with Bigley's mother.
Bigley, of Wadsley Lane, Sheffield, was told he must serve at least 22 years in jail.
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Hull Crown Court heard Bigley was still wearing his heavily blood-soaked clothing and attempting to wash the blood from his hands when was arrested in the early hours of 28 May.
Det Ch Insp Craig Jackson said: "This was a violent and brutal attack on Matthew, a young man whom Bigley once considered his friend.
"The court heard how both he and the victim knew each other for many years, but their friendship broke down after Matthew began a romantic relationship with Bigley's mother.
"His anger and frustration came to the fore in the early hours of that morning in May and a young man who had everything to live for has now tragically lost his life."
Mr Wilson's family said in a statement they were "satisfied" with the jury's verdict.
However, they added: "While we regard the sentence Bigley has been given as fair, no sentence will ever bring Matt back to us.
"We too have been handed a life sentence."
Three vehicles could have been used in a robbery involving two security guards carrying out cash transfers at a bank in Argyll, police have revealed.
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A man has been jailed for life for murdering a former friend who was having a relationship with his mother.