~/.config/gtk-3.0/gtk.css:
VteTerminal {
    padding: 1px 3px 1px 3px;
}

Expected: 3px padding on left and right.
Actual: 1px padding on left, 5px padding on right.

Give an even bigger padding, like 50px. Notice that instead of using that area for padding, it's occupied by actual character cells (as many as fit, the remaining is padding), resulting in a larger than 80x24 initial size in vteapp. The logical size keeps changing as you increment/decrement the font size.

padding:0 causes a 79x23 initial size, 1px top and left padding, charheight-2px and charwidth-2px bottom and right paddings, and vteapp's window shrinks to minimal width as soon as you try to resize it.

(First guess: most of vte uses the actual padding taken from gtk.css, but there's somewhere a component that uses the hardwired 1px value instead.)

(git master, cbc73b6)