Which condition made the western roman empire vulnerable to attack?

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.

the lines on a surface map are called isobars. isobars are lines of constant pressure which are measured in units called millibars. the numbers indicate the amount of air pressure, in millibars, that each line represents.