Installing HikaShop on a website won't stop VM to work. Same for the import.
1. Install HikaShop on your Joomla 1.5. Make sure that you have the same version of HikaShop on both websites.
2. Import from VM via the menu Products->VM. The attributes won't be added to your products in HikaShop. You will have to add them again.
3. If you already exported your database from 1.5 to 2.5, check that the ids of users are the same in your old and your new website. If that's not the case, I would recommend to not export the hikashop_address and hikashop_order tables in the next step.
4. Go in the database of your Joomla 1.5 website via phpmyadmin and export all your hikashop tables.
5. Import it in the database of your Joomla 2.5 website.
6. Copy the folder /media/com_hikashop/upload from the old to the new website.