Many of us have houses with no VCL below roof-void level. That void will probably have been empty, and then we fill it with 6, 9, 12 inches of insulation. Even then, if we keep the 'ventilation path' (in an old building the bits where the roof does not quite touch the walls, rather that specific 'vents') open we probably still don't have problems. If as you say, the lay-up is new I would, for the sake of a ha-p-orth of ... VCL, use a VCL, and I would probably use an intelligent membrane. A well-taped plastic sheet would do. Sticking my nose in, have you thought of 400 or 500 glass wool, for a relatively small marginal cost? If you install 300, you will be very unlikely to top up, I suspect, whereas if you do it from scratch you've got the gains, right from the start.

A VCL prevents (or reduces) the amount of moisture leaking from the warmer internal space of the dwelling to the colder space of the ventilated roof area. Whether that is a problem depends on whether the insulation is susceptible to damp, as well as the magnitudes of the differences in temperature and humidity. So for example foam insulation like EPS beads is unlikely to be affected, whilst rockwool may become saturated near its top and cease to function, and organic insulation may start to rot in time.

I would always use a vapour membrane on the warm side of the insulation and more crucially use as the air tightness barrier too, joining it to air barriers in other building elements.