Conversation so far:
what is the purpose of system vision document
A system vision document is a document that defines the high-level scope and the purpose of the system or project. This helps in reducing the risks of the system.
what components are included in a system vision document
System Vision Documents include the problem description, the anticipated business benefits, and the system capabilities.
when should a vision document be made
The Vision is created early in the Inception phase. It should evolve steadily during the earlier portion of the lifecycle, with changes slowing during Construction.
are system vision documents similar to business cases
A Vision document provides input to the project approval process and is, therefore, closely related to the Business case. It communicates the fundamental why and what for the project.

Question: how lengthy is the document
Self-contained rewrite: how lengthy is a vision document