Maya 2018 | V-Ray 3.6
By default V-Ray for Maya, when Production Engine is set to CUDA (GPU), resizes all the textures to 512×512 in order to save GPU memory.
Naturally, this may cause the textures to appear blurry and lacking detail in the rendered image.
To avoid this,
In Render Settings > V-Ray tab > Production renderer,
Set GPU Resize Textures to Full Size Textures, so the textures will not be resized at all (but consume more memory),
Or On-demand mipmapping, so the textures data will be optimized to reduce memory consumption (but not quality) in a pre-render process.
To get full size textures in IPR mode, do the same in the IPR tab