You can enable the product review like the given navigation below:
For Joomla 1.5 and virtuemart 1.1.9 stable the navigations are:
Go to components->com_virtuemart->configuration->global->Auto-Publish Reviews?
Just enable it. You have your solution. You can refer this image :
For Joomla 2.5 and virtuemart 2.0.16 stable the navigations are:
Go to components->com_virtuemart->configuration->shopfront->Auto-Publish Reviews?