A vaandrig is an ensign, the lowest officer rank in the army. From the 1800s, the term is more specifically used for officers-in-training. The rank was also used in civil militias.
You may come across the term in a range of records, including military records, church records, and town records.

Citizen militia of Amsterdam, circa 1770-1783: Captain, Ensign, Sergeant, Militia man. Credits: Vinkhuijzen collection, New York Public Library (public domain)