If U.S. patented technology is built into a vehicle in a foreign country, would it be considered patent infringement if that vehicle later traveled into the United States?

Not necessarily. 35 U.S.C. 272 states: 

"The use of any invention in any vessel, aircraft or vehicle of any country which affords similar privileges to vessels, aircraft or vehicles of the United States, entering the United States temporarily or accidentally, shall not constitute infringement of any patent, if the invention is used exclusively for the needs of the vessel, aircraft or vehicle and is not offered for sale or sold in or used for the manufacture of anything to be sold in or exported from the United States" (emphasis added).

Note that this exception only applies to countries offering reciprocal privileges. 

Thus, if a United States patent covered a design for landing gear, and a Japanese airliner with the claimed landing gear flew into the United States to deliver passengers, the use of the patented landing gear in the United States would not infringe the U.S. patent.