There must be many, many e-commerce websites that use the combination or Joomla 1.5.x and Virtuemart 1.1x. They are a popular combination and work well. The problem is that these systems are at the end of their life: there will be no further releases even for security patches. Ideally website owners should be migrating now (if you have not already) to Joomla 2.5 and Virtuemart 2.x.
However this is really not a job for the fainthearted, and many website owners will be tempted to stick with a system that works and use a security tool such as sh404sef, Akeeba admin tools pro, or RS Firewall, in the hope that will be enough to protect their site. It may well work for you in the short term at least.
We decided not to do that, but to take the plunge and migrate. We are Joomla developers ourselves and we felt therefore that it would really send the wrong message about our work if we were to continue to use an outdated system. The longer we left the migration the harder it would likely be. Also we wanted to continue to keep abreast of the pace of Joomla development.
So this is how we did it. I will give a brief account of the process that we followed, in the hope that it will help others in a similar situation.
1) Firstly, and I think this is very important, we planned carefully, and tested extensively. No-one should just go ahead and try to migrate a live site, you will cause yourself no end of problems.
2) After testing we decided that the best way to proceed was to migrate to both Joomla 2.5 and Virtuemart 2 simultaneously. We started by creating a new blank Joomla 2.5 installation in a testing folder on our site, and installed our chosen template.
3) We used SPUpgrade to migrate Joomla articles, users, menus and modules from our old site database. We think that this component offers a better solution to migrating Joomla than the jUpgrade component, because it keeps the old ids. Because ids are the key to generating Joomla URLS it keeps your URLS intact, which is important for search engine optimization. Also it leaves your old site database unaffected, so you can always go back to your old site if things go wrong.
4) SPUpgrade doesn't migrate any extensions itself, it just migrates the extensions tables in the database, so the next step was to reinstall our modules. Once we had done that, the module instances already existed with all the parameters intact, so it was just a question of assigning them to their correct positions. That was the Joomla part of the migration done in a few minutes.
5) We wanted to keep our Kunena forum entries. Fortumately SPUpgrade allows you to migrate tables directly from the old database to the new, so we did that with our Kunena tables, then installed Kunena on the new site. Again, it was quite quick.
6) Now it was time to migrate Virtuemart. We migrated the old Virtuemart 1.1x tables to our new database, then installed a new, blank installation of Virtuemart 2.
7) We used the Virtuemart migration tool to import the old products and users into Virtuemart 2. To use this tool you first need to enable the database update tools in the Virtuemart configuration, then open the tools->tools and migration->migration tab. We found it best to migrate everything at once, otherwise there seems to be a problem with matching products to orders. We checked the 'rewrite order numbers' box, and entered the storeowner id (which most likely will be your own admin user id), but left the other options alone. This worked first time, so we didn't try experimenting with these options.
8) The site was ready to be moved. So we made a backup of our old site (just in case), copied the files for our new site to our public html folder, and we were done, barring of course the inevitable minor glitches.
In fact we did do a good bit more testing than that, but these are the essentials of the process that we followed. Once we had figured it out it actually went quite smoothly.
The main issue we faced as a site was due to the fact that we sell downloaded products. They are no longer supported by Virtuemart directly. There is a Virtual products plugin available in the new Virtuemart extensions store, which we tried and found quite inadequate and not secure. Also the license it was released under made it impossible to modify to our needs or even apply a security patch! Eventually we spent some time developing and testing our own plugin which we think is more suitable to our store. We hope to make the plugin available eventually under the GPL.