This is possible using Vertical Mapper - which you stated was used to create the grid in the first place. Here is an exerpt from the Vertical Mapper 3.0 Help file:
"The outer margin of any grid file can be trimmed using a predefined polygon created in a Map window. This tool is particularly useful for trimming grid files to a standard map neatline. Only one grid file can be trimmed at a time.
Choose the region that represents the trimming edge. In the Grid Manager, click the Tools button and choose the Trimmer command. In the Save Trimmed Grid As dialogue, type a new file name for the trimmed grid in the File name box. Once the .grd file is created, it appears in a Map window with a default colour palette applied. You can change the colour range assigned to the grid file. For more information, see Using colour in grids."
Alternatively, if you don't have access to Vertical Mapper any more then you can convert the VM GRD file into a MapInfo MIG file using the 'Grid Tools' utility from the Tool Manager, and then use the MIG Crop function available in the MIG Tools utility which can be downloaded here: MIG Tools