Most Joomla! site developers use the Firebug tool in Firefox to examine displayed pages, to show page HTML and CSS code, and to test the effects of changes within the code. This is a very good tool for the rapid identification of VirtueMart template layout issues and the testing of changes to CSS - without making any actual alterations to the underlying code.
Having identified any required alterations or additions in CSS, those changes can then be written into the relevant CSS files in the Joomla! template or VirtueMart theme CSS, using template/style overrides as necessary.
To see an introduction to the use of Firebug see http://docs.joomla.org/Tutorial:Using_Firebug_With_Your_Joomla_Website