Biden called Poroshenko, a US official said, as the Kiev government and separatist rebels accused each other of bringing down the plane, carrying 295 people.
The official said that Biden spoke to Poroshenko from Detroit, where he is travelling.
"On the call, the vice president offered US assistance to help to determine what happened and why," the official said.
President Barack Obama, who was also on the road, in Delaware, earlier said the crash looked like a "terrible tragedy" and said Washington was trying to establish whether there were any Americans on board the plane, which was flying from Amsterdam to Kuala Lumpur.