Learn how to install Joomla! on your own PC to avoid making mistakes with a Live website. Installing in a test environment lets you see what your Joomla! powered website will look and feel like before you even purchase hosting. You can even install VirtueMart on your local computer and test functionality of the extensions like theme, payment gateway, modules etc before you finally upload the same on the live server. 

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The short answer is no, not at the moment. Currently, the multi-vendor feature is in alpha and should not be used on a live site as it is.

If this feature is important to you, then either wait for a later version of Virtuemart when the feature is stable, or if you need it sooner it will need a great deal of attention from a developer.

Reference: FAQ - VirtueMart Official Forum

Starting with version 1.1, VirtueMart offers to style the shop using themes and templates. A theme is what defines certain parts of the look and feel of your shop. All installations of VirtueMart should start out with the default theme. You can change the Theme in the "Site" section of the configuration form. Usually there's only one theme available, so you can't switch. A good way to think of themes is as "plugins", which contain a collection of page templates, images, stylesheets, javascript and other files. While editing these files can be a bit technical, the layout is separated from the content allowing you to make and distribute your own themes.

Theme Configuration

Themes can be configured for better usability. Configuration parameters can be used to turn on and off certain parts in templates. The configuration parameters are defined in the file /components/com_virtuemart/themes/THEMENAME/theme.xml and follow the common parameter syntax for components, modules and mambots as used in Mambo >= 4.5.1 and Joomla! (Read more about the mosinstall parameter XML syntax). When a user chooses to configure a theme (Shop Configuration ? Tab "Site ? "Layout", these parameters are parsed and displayed in a nice form. Now the user can make changes to the configuration. The current configuration values are stored in the file /components/com_virtuemart/themes/THEMENAME/theme.config.php. This file is renewed with the new configuration values each time a user saves the theme configuration.