According to http://www.viceteam.org/plain/NEWS "Vice-1.19 will be the last Gnome1/Gtk1 version officially supported." The next version will use Gnome2. There is an unofficial Gnome2-patch for vice-1.19 on the vice hp. Might be a nice idea to include that patch in Gentoo. Will post a patch.
Created attachment 84833 [details, diff] vice-gnome2-patch This patch add a local USE flag gnome2 to the app-emulation/vice-1.19 ebuild. The patch is only applied if both the gnome and the gnome2 USE flag are activated.
Since you posted, Vice 2.10 has been released, which officially supports GTK2 (and drops support for GTK1)
Er, that's supposed to be version 1.20, not 2.10 ;)
yeah, we'll skip the unofficial patch for 1.19.