Joomla/PHP/MySQL/cart guru needed for site tweaks

We have two issues we need to deal with before we launch our new Joomla site. First, we are trying to launch a site but are concerned about load and our sites ability to handle hundreds to thousands of simultaneous users. We have looked into components JMS Multi-site and Synk but neither seem to get us exactly what we're looking for. Hopefully, you will know the best way to scale a Joomla site and help us with the implementation. We'd also like failover for both the site and database. The second issue issues is getting our Virtuemart working without crashing our site. Maybe Virtuemart isn't the best solution? Maybe we've configured something wrong? We then need products displayed on a Sobi template. This may take custom PHP code?