Whilst vermiculite is the right product for the job, think of the future, flexi liners are designed to last 10 years, that said, depending on type and quality and humidity of the wood, the condensation generated can be very corrosive, so liner may need replacing in as little as 5 years, it may also last 15 years.
In addition, do you know the source of the vermiculite as material of this age may contain asbestos?
Have you thought of installing a bespoke insulated flue, this may have a longer design life.
