For those looking for a direct answer... No, most camper vans do not require one.
There are a few reasons why vapour barriers are not necessary the first being is a vapour barrier, when put in houses and buildings it is there to stop vapour passing through permeable surfaces, it stops damp and ruin to household insulation as the barrier is a waterproof closed cell material.
The Thermo liner (insulation) we recommend from deadening.co.uk is closed cell layer with a reflective foil finish. This product along with your other insulation will reduce the extreme difference of temperature between then outside and inside temperature and act as a barrier between the outside and inside temperatures.
The thermo fleece liner is also designed in a way that it does not lose its insulating values when damp. Quoted directly form the manufactures spec "It will not absorb moisture in high humidity conditions and does not support mould growth."
Having the correct ventilation
To keep the condensation to the minimum ventilation is key! Having an opening window or a ceiling vent will allow for the condensation to dissipate accordingly and keep the moisture and condensation from gathering on surfaces. These methods are much more effective than a home made moisture barrier.
If you are building a camper van for extremes in temperature and conditions then contact your parts supplier or a camper van conversion specialist for advice and guidance. If you would like to purchase sound deadening and camper van insulation find it via the link below.