The would-be swindlers, wearing business suits, tried to convince Swiss Guards at a Vatican City gate earlier this month that "cardinals were expecting them".
Police said the suspects, a middle-aged Dutchman and a US citizen, were detained by Vatican authorities after rapid checks by Vatican officials showed they had no such appointment nor connections with the Institute for Religious Works, the formal name of the bank, which is behind the tiny city-state's walls and is not open to the public.
The Vatican has been scrambling to upgrade procedures and standards at the bank since a 2010 money-laundering probe.
Officers said the fake documents purported to be bond certificates for non-Italian companies.
Investigators suspect the men might have planned to use the fake bonds as security to open a hefty line of credit through the Vatican bank. The Vatican asked Italian authorities to help in the investigation.
Both suspects, whose names were not released by police, had been previously investigated for attempted fraud in Asian countries, police said.