The honest test is not headcount or revenue, it is whether roadmaps exist. If your clients have no agreed technology plan beyond the current quarter, every renewal is a fresh argument about price, because there is nothing else on the table. A vCIO exists to put something else on the table.
The second test is whether the role is real. A vCIO who cannot hold a budget conversation with an owner, connect technology decisions to business outcomes, and say no to spending that will not pay back is a title on an org chart. Giving the label to your most senior technician usually produces meetings rather than strategy, because the skills that make someone excellent at solving problems are not the ones that make them credible discussing capital.
Many MSPs start by carving the function out of the owner's week — a fixed day, a defined client list — before hiring for it. That is a reasonable first step and it surfaces the real question quickly: whether clients will pay for the conversation, or only for the fix.