2018 Vauxhall Vivaro just bought 2018 vivaro 16cdti starts up fine no smoke but after half a mile white smoke comes out of exhaust no power loss .

Could be coolant or fuel , normal smoke on start up should clear as engine reaches working temp . You need to keep an eye on coolant level and see if it drops or cooling system over pressurising . If it's over fuelling due possibly to injectors , you can usually smell it at rear exhaust , May even be fuel filter needing changed