What is vygotsky's term for the level at which a child can almost but not quite understand or perform a task?

The term is "zone of proximal development".

ZPD which is an abbreviation for the zone of proximal development is the contrast between what children or learner can manage without help, and what they can't do. The idea was presented, yet not completely created, by Lev Vygotsky amid the last decade of his life. He was a psychologist born in 1896.

Zone of proximal development

This refers to the difference between what a learner can do and what they cannot do without assistance. It is the capacity of the learner to accomplish a task without requiring assistance and depicts the level at which they can understand or perform the task