"Why didn't the American people have a better sense of his health picture? Why didn't the American people have more accurate information about what he was actually dealing with? This is serious stuff," Vance told reporters as he wrapped up a trip to Rome.
He wished Biden "the right recovery".
The remarks by Vance, a Republican, captured the renewed focus on the health of the 82-year-old Democratic former president with the publication of a book that details widespread concerns about Biden's mental acuity among aides and Democratic insiders as he pursued re-election in 2024.
Excerpts from the book have prompted new questions about whether critical information was withheld from the US public about Biden's ability to serve in the White House.
Biden's closest aides have dismissed those concerns, saying Biden was fully capable of making important decisions.
A spokesperson for Biden did not immediately return a Reuters request for comment.
Biden has appeared on television to reject accusations that his mental capacity had diminished during his 2021-2025 term.
"There's nothing to sustain that," he said on ABC's The View on May 8.
Biden, the oldest person ever to serve as US president, was forced to drop his re-election bid last July after a stumbling debate performance against Republican rival Donald Trump eroded his support among fellow Democrats.
Biden's vice president, Kamala Harris, launched a bid of her own but lost to Trump in the November 2024 election.
Biden's office said he had been diagnosed on Friday with prostate cancer that has spread to his bones.
Several doctors told Reuters that cancers like this are typically diagnosed before they reach such an advanced stage.
"I would assume the former president gets a very thorough physical every year," said Dr Chris George, medical director of the cancer program at Northwestern Health Network.
"It's sort of hard for me to believe that he's had a (blood test) within the past year that was normal."
Dr Herbert Lepor, a urologist at NYU Langone Health, said that given the available screening options, "it is a bit unusual in the modern era to detect cancers at this late stage".
About 70 per cent of prostate cancer cases were diagnosed before they spread to other organs, according to the Centers for Disease Control and Prevention.
US guidelines do not recommend annual blood screening for men over 70 and it is unclear whether the annual presidential exam would have included those tests.
A new book, Original Sin, by journalists Jake Tapper and Alex Thompson put a spotlight on Biden's mental acuity in his final months in office.
"It was a mistake for Democrats to not listen to the voters earlier," US Senator Chris Murphy of Connecticut, a potential 2028 Democratic presidential candidate, said on NBC on Sunday.
Biden faced no serious challenge for the 2024 Democratic nomination, and party leaders repeatedly vouched for his ability to serve a second four-year term even though a Reuters/Ipsos poll suggested 74 per cent of voters in January 2024 thought he was too old for the job.
Biden's cancer diagnosis drew an outpouring of sympathy from supporters and rivals alike, including Trump.
Biden thanked the public on behalf of his wife and himself for their support in a social media post released early on Monday.
"Cancer touches us all. Like so many of you, Jill and I have learned that we are strongest in the broken places. Thank you for lifting us up with love and support," he said.
