I will assume you are discussing a vapor barrier placed under a bedding material onto which a concrete slab is cast. I'm not familiar with your terms "DPC" and "surfacebed".
You must first understand the need for a vapor barrier. It is two-fold. First is to provide protection against excessive loss of moisture from the concrete during the initial set and curing into the subgrade. If you place the vapor barrier beneath the bedding layer, this will not be accomplished unless you wet the bedding soil before placement, which can cause a few other problems.
Another reason for the vapor barrier is to prevent vapor migration from the soil into the concrete slab. It makes no difference where you place the vapor barrier (directly under the slab or under the bedding layer) for this purpose; however, I'm not convinced that there is a great deal of detrimental vapor transmission into the slab from the soil unless the water table is within a foot or so of the slab bottom.
Once you understand the purposes of a vapor barrier, you can then decide whether it is needed and where it should be placed to best suit your project's needs. Also, you might consider that in some areas, a vapor barrier is required by the building code or at the least perceived by the inspectors to be required. Check you local code requirements.
Vapor barriers can cause numerous problems in the construction of slabs and in the performance of slabs. Be careful how you specify them and how they are allowed to be placed and protected at construction.
