Ok, I understand.
Probably the problem is because you have this rule:

\?option=com_virtuemart&page=shop.browse&category_id=3[0-9]

But I guess you also have this:

\?option=com_virtuemart&page=shop.browse&category_id=30[0-9]

Both matches with URLs associated to the second rule, but the first one is applied cause both match and ReDJ uses the first in ordering. You can solve adding a dollar at the end of each rule:

\?option=com_virtuemart&page=shop.browse&category_id=3[0-9]$
...
\?option=com_virtuemart&page=shop.browse&category_id=30[0-9]$

Kind regards,
Luigi

Ok,
but the rule should redirect this:

To this:

And it works, but is redirected (again) to this:

Are you sure the destination exists?

OooooKeeeey,
now is clear. The rule doesn't return the product id. Looking at the rule again it seems there is a ) more then needed in the preg_match macro. Try to change:

Into:

Let me know,
Luigi