A rendered image is only pixles, so to print the image the computer needs to know how big those are going to be on the paper, that is where dpi comes in, how many dots (pixles) are going to be used per printed inch.
In photoshop you can alter the dpi value and choose to resize the actual pixles, or just alter the dpi value without resizing the actual image. If you just alter the dpi value you will see that the width and heigh values in inches (mm or pt depending on your setup) are automatically adjusted accordingly.
Some background info:
72dpi is an old default used back in the days where most computerscreens were crt and had about 72 pixles per inch in physical size, and this default is added to an image when saved when no setting is applied from the application. As far as I know V-ray does not give you an option to set another value. This was normal for most renderers back in the days since all they do is making colored pixles and the user had to alter this when and if they wanted to print the image to a fixed size.
My rule of thumb is to use 100 dots per cm
(I don’t use inch as a measurement since I am in Europe)
And that makes the math easy to do in the head, an A4 paper size is 29.7x21 cm and therefore needs 2970x2100 pixels. This gives a print resolution of 254 pdi which can not be told apart from 300 dpi by the naked eye.
Good luck and happy rendering and calculating