Vietnamese airline VietJetAir has finalised a huge order for 100 Airbus planes, mainly mid-range A320s and A321s, in a contract that could be valued at over USD 9.0 billion, Airbus said Tuesday.The 100-plane order, which includes a small number of leasing options, was inked in Singapore on Tuesday.Initial negotiations were carried out several months ago, culminating in a firm order by VietJetAir last month to purchase 62 A320s for over USD 6 billion. A second round of talks led to the expansion of the order list. No official value has been released for the contract and estimates are based on list price for the planes.VietJetAir is the country's first private airline and it started operations in 2011. It already operates a fleet of 11 Airbus 320s on destinations in south-east Asia and southern China.