He said he felt “the whole notion that there's still this romance with Putin” was worrisome, but declined to comment further on an FBI probe of possible links between Russia and Donald Trump associates.
“What in the hell are we doing?” the Democrat asked.
Mr Trump's frequent Twitter comments are “not helpful”, he said, adding that the US leader needs to understand that “the President's words matter a great deal” and have global repercussions."
He said: “I hope the president will realise the campaign has to end and he's got to start to govern,” Mr Biden told reporters after a Democratic rally against the dismantling of Obamacare. The event was Mr Biden's first public appearance since leaving the White House."
Along with Barack Obama, Mr Biden said they had tried to give Mr Trump “an opportunity to get his sea legs” as president.
But he said it was "getting down to the wire for whether (Trump) is really going to take charge” as president.
He added that he didn't know if Mr Trump's comments about foreign policy showed “a lack of understanding of government and how international relations work” or were a reflection of “a policy prescription that's changing.”
Either way, the American people should be "concerned", he said, adding that the United States as “the most powerful, most respected nation in the world" was "sounding a very uncertain trumpet”.
When the US projects uncertainty, “a lot of stupid things can happen as a consequence,” Mr Biden said.
On healthcare, Mr Biden said a Republican bill being considered in the House to overhaul Obama's Affordable Care Act was a huge transfer of money to the super-wealthy.
The biggest beneficiaries are millionaires who would admit, if asked, that they don't need the money, he said.
“Ask them if it's fair. They'd say no,” Mr Biden said.
Additional reporting by Associated Press