Vector support could be kind of like a "smart layer" in photoshop, you double click it to open it, but instead of incorporating vector abilities into PDN, just open an SVG editor like Inkscape. Then when you save and close Inkscape, the layer in PDN is updated.
The layer itself would only contain the xml text info, and PDN would need to render it - thus PDN would need to utilise an "svg viewer" type plugin.
The user would also need to have Inkscape installed.
I thought this would be the best easiest way to do it.
(There is no need to reinvent the wheel)