I am going on the master cleanse diet, then going to juice fast, until April 16th (my birthday)

but I also want to go out on my birthday and drink. I would like to drink skyy vodka, mixed with sugar free red bull. Would I gain any weight back? Also, would it harm me or just get me drunk really fast? I would only drink a very small amount, until I am buzzed. Dont critizise my fasting or drinking, just give me the facts please.

Thanks <3