BanquetTables, you are talking about vm1.1.x series. In vm1.5 has no theme directory any longer.
VM1.5 can do that exactly, but k0nan seems to went a different way than I expected.
i'm using joomla ja_purity as default template.
i've assigned to every virtuemart view the ja_purity template.
I fear there is the problem. You assigned every view with joomla to use the ja_purity template. But this wont help people, because they want different templates/layouts by category id or product id.
To be sure, you use it the intended way do the following. Remove all configuration within joomla for the templates. Just choose a standard for your joomla page. Now you should see the page and joomla in the choosen standard template.
When you enter now the virtuemart configuration, you can set a standard template for all com_virtuemart things. Not per view, just for the whole component. To extend this, you can then choose a different standard template for categories and products.
When you save now, then you should be able to see the extra layouts in the selector, of course you need an extra file placed like you described. When you do not see the extra layouts, something is wrong.
The selector shows the default layouts of the view and adds the extra layouts saved in the template.