Anyone been doing the same research as me . . if so, hope this is helpful.
In the absence of any transport after the game to get back to the airport then I've come to the conclusion that apart from taxis the best option might be to walk to the Feria de Madrid metro station which is on the same line as the airport terminals, so from there it's just a direct 10-minute metro journey. Trying to get back into Madrid will be busy and also requires changing lines to get back on Line 8 to the airport, whereas this should be quieter and quicker.
There are two basic routes walking to that metro station, both taking about 45 minutes.
The slightly quicker one is to go North from the stadium along Av. de Luis Aragones for about a kilometre and then cross the main roads by walkways and then follow the edge of the Parque Juan Carlos for about another kilometre. The problem with this route could be it's on the Tottenham route back to their metro terminal (Canillejas) and then after getting over the roads it could be a bit deserted at night by the park.
The other option is somewhere along Av. de Luis Aragones go left and work your way North West along the main streets for a couple of kilometres, crossing the two main roads along the way. The advantage of this route might be more folks around, taxis available if you want to give up, and also it goes away from the Tottenham route to Canillejas.
Anyway, I'm on the 3.05am Vueling out of Terminal 4 so should be plenty of time to get there, but if anyone else has been looking at how to get to the flights in the early hours and has any constructive feedback I'd be interested to know what others are planning.