If the vacuum oven has condensing humidity on its walls at 50 ºC, you have to account for the contribution of vapour pressure of liquid water at that temperature (92.5 mmHg), meaning that you are attempting to dry the apple slices at a rather coarse vacuum level, while the atmosphere of the vacuum oven reached saturation in terms of relative humidity. Also important to be considered for the efficiency of drying, is how heat is transferred to maintain the mentioned temperature.

we used to put desiccant on a tray eg silica gel

May be you'd better check the capacity of vacuum oven. Very likely it was because the amount of water to be removed from apple slice is  beyond the capacity of vacuum system?

I agree with Carlous that the temperature and pressure should not reached at saturation level of humidity causing condensate. Make arrangement for dehumidifying the chamber by technically or putting desiccants.

Sir

I agree with all. Offcourse air will get saturated as fresh fruits have high moisture. I am doubtful about keeping desicant as., it is also getting heated at Same temperature in maintained vaccume and will get evaporated and condensate on walls. Eventhough you can try. Replacing air frequently with dehumidified air or cleaning walls will also be useful though it intercepts the drying and consumes time.

Another possibility is that too much sample is dried in a single run. I recommend to reduce the amount of sample in the oven (with or without drying agent) until no more problems arise. 

I think you need to desiccants such as Activated alumina, Potassium carbonate, Sucrose and Silica gel because you know that a desiccant is a substance that absorbs water. It is most commonly used to remove humidity that would normally degrade or even destroy products sensitive to moisture. Good luck my brother

The vacuum oven is use to remove large amount of moisture from plant tissue or soils. You need to have a vacuum system that has a sump with a drainage easy to release the condense water.