That's just a warning message informing you, that has been reached the actual view-port limit and not all items will be shown in the view-port.
I would suggest you to check you are using presets that are meant to cover a large area(s). These will create fewer instances and are less taxing for the renderer. For both the Lawns and Meadows presets you will see there are two versions for each type, one with a (detail) suffix and another with a (large) suffix. Unless you're filling a small area, I'd pick the large ones.
To improve the overall render performance, an option would be to reduce the GI quality for the grass. To do it, select the Forest object->right click->V-Ray properties->set GI subdivs multiplier to 0.1 (or other lower value). This change may cut the final render time by 30%.
Also, some material simplification will drop down the final render time significantly. The grass material used in our presets is quite complex to give a nice realistic look but it can be simplified which will give you shorter render times. To cut render-time, you can remove VRay2SideMtl from materials and use only the front VRayMtl. Grass will look less realistic, but you'll notice a great performance improvement.
Please don't forget to double check, if the Dynamic memory limit is set properly (you can set Dynamic memory limit to 0 to remove any limit in that case, your renderer will take as much memory as needed). This parameter has a great impact in performance when rendering complex scenes.
Hope that helps.