HI sood15,
Tessa is correct, however it should work either way as long as you have upgraded Viretuemart at least to 1.1.9 before you try to upgrade Joomla.
The way I did it was to upgrade to VM 1.1.9, then upgraded Joomla from 1.5 to 2.5, then completed the migration step.
Since VM 2.0 works in both Joomla 1.5 and Joomla 2.5, it shouldn't make any difference if you do the migration first then the Joomla upgrade. I did't try it that way, but there is no reason why it wouldn't work.
Just be sure to use a test installation when you try it. Let us know which way you decide and how it goes. Ask questions if there are any problems.
Cheers
Ed