Follow the steps below:

1. Login to the back-end administrative control panel
2. Before starting to create menus, you need to know the Category IDs of the product categories that you want to make into menus. Go to Components > VirtueMart > Category Tree. An id of a product category is represented by the numbers listed under the title: id. Memorize the ids of those product categories that you want to make into menus
3. Go to Menus then select the Menu Module where you want the menus to go under
4. Click the New button
5. From the Menu Item Type list, click VirtueMart
6. Give it a Title and a Parent Item if necessary then adjust the other settings
7. Click the arrow beside Parameters (Component) once to expand it if it was not expanded
8. Type the Category ID into its designated field
9. Click Save once to finish