VirtueMart has a lot of plugins on board. They are ususally installed by the AIO (all in one) installer. When you check for updates within joomla, all plugins are also listed there in case they are outdated, but we do not provide an extra installer for every plugin. It is a lot work and almost no one needs it.
But sometimes it may be useful to update only a single plugin of the AIO. In this case the simplest method is to locate and download the version you need from http://dev.virtuemart.net/projects/virtuemart/files, extract the AIO and open the path /admin/plugins. When you want to update for example the paypal plugin, just copy the folder /admin/plugins/vmpayment/paypal into your installation. Thereafter open the paypal payment method in your shop's backend and store it one time in order to update the database tables (just in case).