Oh god, I didnt realise other people in the rest of the world wouldn't understand the common usage for the 'V' sign- to stick two fingers up at someone over here is to tell them to 'F*** off' or words to that effect (for Spike, it'd be 'sod off' cuz thats the sentence he tends to use the most).
But thats definitely what it is- Xander is blaming Spike, so Spike says 'oh sod off' with a hand gesture- a very Brit thing to do nowadays 🙂 I like that they've included that then, if its not something common to all countries- it means Joss is really picking up on the smallest details on Spike and making them British!
Its so adorable that others don't know what it means!! Aww
Edit:
Oh and there's a theory that the gesture comes from the battle of Agincourt.
The French would capture British archers and chop their middle and first fingers off on their right hand, to stop them using a bow and arrow.
So, at Agincourt, the British archers all stuck up their 2fingers at the French in a 'haha look we have our fingers' sort of way.
The gesture was kept and is now used in a 'feck off' way in modern times.