The move will support nearly 100,000 jobs in the US aerospace value stream.
Decision paves the way for US to seek $10bn in annual retaliatory tariffs on EU imports, Boeing says.
ULA is planning to cut another 400-500 jobs in 2017, according to its CEO Tory Bruno.
The KC-46's will be able to refuel the C-17 cargo plane, the F-16 fighter and the F/A-18 fighter jets.
The deal marks a huge win for Boeing as VietJet has until date only purchased and operated Airbus airplanes.
The factory will be used to manufacture wings for Boeing 777X, the world's biggest passenger airliner.
Defence Secretary Michael Fallon said the UK would order a P-8 Poseidon jet in the next few months.
Bombardier received a key order for its narrowbody planes last week from Delta Air Lines.
New company has yet to deliver a seat but is run by industry veterans Tom McFarland and Jim Downey.
Nine Boeing P-8A Poseidon planes being sold to the UK military.
The new jets will have sophisticated communications equipment and upgrades such as anti-missile devices.
Aviation giant says it will take a $569m after-tax charge on its fourth-quarter earnings.
European company had 1022 orders by the end of December 2015, compared to Boeing's 743.
Downsizing partly blamed on freeze of US Export-Import Bank funding.