[2] Vahl participated W. A. Graah's expedition to uninhabited areas of East Greenland in 1828–1830 with the purpose to search for the lost Eastern Norse Settlement.
So he travelled in West Greenland from 1829 to 1836, visiting all the Danish colonies from Julianehåb in the South to Upernavik in the North.
The Vahl collections added several lengths to the previous investigations by Paul Egede, Morten Wormskjold and others, and effectively laid the foundation of knowledge about the flora of Greenland.
In contrast to his predecessors, Vahl made meticulous notes on the finding circumstances, like exact location and habitat of the plants.
His successor, Johan Lange, picked up the task two decades later, which resulted in Conspectus Florae Groenlandicae (1887–1894).