If you have VirtueMart eCommerce component installed in your Joomla site, then when a customer registers on your site to purchase something, the customer will receive an customer welcome email that says something like ...

"Thank you for registering at XXXXX. Your customer account has been created. You may login to XXXX using the following username and password: XXXX XXXXX"

If you want to edit the text of this email or perhaps remove the display of the username and password, the file that you would need to change is ...

/administrator/components/com_virtuemart/languages/common/english.php

Look for the array key "PHPSHOP_USER_SEND_REGISTRATION_DETAILS" and change the text associated with that key.

To change the "order confirmation email", see this tutorial.