It's a direct parody of the "Miku Miku ni Shite Ageru(Shiteyanyo)" Vocaloid song. But rather than leave in "Miku miku" it says "hebi hebi" which means "Snake snake."
I'm not entirely sure what is being said, though. The translations of the song seem to differ on the wording. The closest translation could be "I'll make it (you) snake-snake."
Um metal gear and vocaloid arent unlikly crossovers anymore seeing how the vocaloid tool is in peace walker along with vocaloid voiced bosses... but big boss doesnt wear that camo so maybe it's still unlikly
