Final answer:
Vlad II Dracul's main duty as a frontier captain under King Sigismund was to defend against invaders, particularly from the Ottoman Empire.
Step-by-step explanation:
Vlad II Dracul's assigned duties as a frontier captain in Transylvania, under King Sigismund of Hungary, included defending against invaders. As a member of the Order of the Dragon, his role was to protect the interests of Christianity by fighting off threats, especially from the Ottoman Empire. This entailed military engagements, fortification of the borders, and maintaining the security of the region. While diplomatic negotiations and tax collection were also significant administrative duties in such positions historically, Vlad II Dracul was primarily tasked with defense.