I tried to rewrite metatags from virtuemart categories with the "manage metatags" tool from jomsef 3.99 on my site, I configured the extension metatag in many diferent options, but every time I ask to update the metatags it clean mine and rewrite an automated one, can you explain me how to configure in the right way the extension metatag to make me able to write my own one, and when its empty, create an auto metatag,

this is the other config.

Hi,

The meta tags are automatically generated when the SEF URL is created in JoomSEF. You can then customize those meta tags manually.
But if you use the Update Meta Tags function, all the current meta tags are regenerated, thus your custom ones are rewritten. It's not possible to automatically regenerate only the empty ones.
You can either use the JoomSEF's URLs Manager to run the Update Meta Tags function only for selected SEF URLs, or you can lock the SEF URLs containing your custom meta tags, so they won't be updated during the meta tags update at all.