Uncertain breadth of meaning.
Vague terms are allowed in contracts because you know the quality of the thing discussed, but you do not know the quantity. Though it is never great to have anything that is not certainly clear in a contract, vague terms are used all the time in contracts. A very widely known example is in a sports contract, specifically baseball. In baseball trades, it is very common to trade for a specific player and another minor player (pitcher) to be named later. This is a vague term. The quality of the thing is clear (a minor play, a minor pitcher), but the quantity (or in this example: identity) is not known.
Vague terms are permitted even though ambiguous terms are not because the vague term does not bring the intent of the parties into question. The intent is clear, so the court can make decisions based on vague terms, where ambiguous terms are not as clear.