Hi,
No, it won't. You can update the Virtuemart and your Joomla. (but before updating, make sure you have the latest version of LightMart)
Check here for the latest version of LightMart:
www.flexiblewebdesign.com/forum-joomla-virtuemart-template-lightmart-support/969-request-lightmart-update
and see the release notes.
LightMart 1.2 -> compatible with VM2.0.22a
LightMart 1.3 -> compatible with VM2.0.26a
Your LightMart must also be compatible with that Virtuemart version, otherwise you will have some layout problems (in the front-end. Your backend will be exactly the same, since LightMart doesn't have any effect in the backend). But i don't think updating Virtuemart will solve this 500 internal server. Because it is obvious, it is a server related issue. Most likely caused by missing write access. (your server fails to generate the PDF invoice)
Also as i have told you before:
This issue can't be related to LightMart, because LightMart is a template for front-end only. It has no effectiveness in the back-end.
We don't provide support for solving the Virtuemart Component related problems. Try to raise this issue in
forum.virtuemart.net
Regards,
Support