At their best they got most of England and the throne, But they didnt get all of it.
To be honest they didnt get much past Birmingham and London actually. They were unable to take the Kingdom of Wessex and kick the Saxons out and by 1066 the Saxons had regained their territory and all but kicked the Vikings out. So in conclusion, The Vikings never took land in southern England, so therefore they were unable to conquer the whole of England.
At its peak the Viking/Saxon boundary of England ran from the Thames Estuary along to the Midlands and what is now Birmingham before turning upwards towards what is today Liverpool, While they did get the crown their kingdom never covered the whole of England and by 1066 the Saxons had all but routed them from England after losing the battle of Stamford Bridge to King Harold, Who then a few days later would go onto lose the Battle of Hastings, and inturn England to the Normans.
They only got the throne when in 1003 Sweyn Forkbeard declared himself king, But that was purely due to their conquests and owning the majority of the land. Problem was that after Sweyn's death the Crown passed back to the Saxons in 1014.