Follow these steps to upload your theme in VirtueMart:
STEP 1: Log in to your Control Panel.
STEP 2: From the top navigation bar of panel, navigate to Extensions -> Manage > Install.
STEP 3: Click the browse button and install the theme by clicking upload and install button.
STEP 4: After uploading the theme, set the theme as your default theme.
STEP 5: Now you may visit the site and can see how it looks on browser.