The easiest way is to start Vice and then go to full screen by pressing ALT+ENTER. After that, exit out of Vice by pressing ALT+X. Then, when you start Vice next time, it will automatically start in full screen. You'll need to use an autohotkey-script to exit properly, using the ESC key. Copy and paste the following code in the "Edit Emulator / Autohotkey-script" section:
$Esc::
{
Process, Close, {{{StartupEXE}}}
}
vk07sc000::
Send, !{F4}
Return