Although for some reason you have posted in the old VirtueMart 1.1 section (which we no lonfger support) I suspect you are trying to install VirtueMart 3? If so you are trying to install on an incompatible Joomla version. VM3 only installs on Joomla 2.5.x or Joomla 3.3.x (eg not J3.1 or 3.2 of J1.5)