If you want to rerender only a part of your image you can make use of the region rendering.
But you can only render one region at a time and these have to be rectangle, loosing a lot of time in some circumstances.
Or you can take your latest rendering into Photoshop, paint a black and white mask on top of it and load that into Vray’s render mask slot, which is cumbersome.
The RenderMaskPainter eliminates this problem by letting you paint a mask directly in the viewport and feeding it into Vray’s mask slot automatically.
With a few steps its possible to mask out several areas in different shapes.
There is also the possibility to change the brush size…
… or to invert the display.
The rendered image from Vray, using the exported mask.