Why do I get loss of detail when I render an object with lot of detail using V-Ray GI?

Loss of detail is very vague term. What do you mean by loss of detail? Does the object get blurred? What type of detail that object have? if you could elaborate on this, I might be able to help you better. But if your image is getting blurred, and you haven’t turned on Depth of field on your camera then try to reduce "Area Blur Value" in Image sample setting in Vray sampling (antialiasing) rollout. It should not be more than1.5, otherwise image starts blurring.