How might the plague have helped make Byzantium more vulnerable to foreign attack?

The plague had several indirect consequences that made Byzantium more vulnerable to foreign attack:

Economic disruption: The plague caused major disruptions to the Byzantine economy, including labor shortages, trade disruptions, and declines in agricultural production.

Weakened defenses: The plague led to manpower shortages in military that made it hard for them to effectively defend their borders. Furthermore, the plague claimed life of experienced military leaders reducing Byzantium strategic depth.

Political instability: Weakened by economic disruption and military losses to the Ottoman and Venetian attacks which increased political instability within the empire contributing to succession struggles and internal conflicts weakening it further.

These consequences combined increased external threats and further the process of Byzantine decline making it harder for the empire to recover to its strength prior to the Justinian's time.