I'm using Wamp5 for a localhost with Joomla! 1.5 RC3 and I kept getting error messages whenever I tried to install VM 1.1 nightly build. I tried the memory fixes, I changed the file upload size in php.ini, I changed to Legacy mode, but no matter what I did, I couldn't get com_virtuemart_1.1.0 to install. The only way I could get it to work was to install it manually. If you're having the same problem, but want to start working with VM1.1, here's how to do it:
1 - Make a new folder in administration/components called com_vm, so now it looks like this: administration/components/com_vm.
2 - Extract all the files from com_virtuemart_1.1.0.zip into the com_vm folder.
3 - Use "Install from Directory" in Joomla! admin and enter this path: ../administration/components/com_vm/ then click install. (notice the 2 leading .. and the / at the end).
After the component is installed, the modules seem to install the normal way.
Hope this helps someone,
Bill