Which condition made the western Roman empire vulnerable to attack?

It was vulnerable because most of the Rome lands were captured and so when a threat came the conquered place joins the threat.

I believe the answer is: Splitting of empire; Weak borders

The splitting empire happened because of the power struggle among the kings' Sons. The split resulted in the reduction of Roman military power which made them become weaker compared to other empires. Weak border provide opportunities for Barbarian tribes to come in an raid Roman villages.