A vampire riddle has been cited in print since at least 1990:
 
Q: Why do vampires drink blood?
A: Because root beer makes them burp.
 
A different punchline has been cited in at least 1998:
 
Q: Why do vampires drink blood?
A: Because coffee keeps them awake all day!
             
   
11 February 1990, Chicago (IL) Tribune, Spots sec., pg. 1:
Q: Why do vampires drink blood?
A: Because root beer makes them burp.
Liam Donoguhe, 4. Wilmette
   
28 August 1994, Chicago (IL) Tribune, Spots sec., pg. 1:
Q: Why do vampires drink blood?
A: Because root beer makes them burp.
Madeline Harrison, 7, Arlington Heights
 
29 October 1994, The Evening Telegram (Rocky Mount, NC), “The Riddle Corner,” pg. 7A, col. 1:
Why do vampires drink blood?
Root beer makes them BU-R-RP.
 
26 February 1998, Los Angeles (CA) Times, “Laugh Lines: Pint-Sized Punch Lines,” pg. E3, col. 3:
Why do vampires drink blood? Root beer makes them burp.(Nicholas Mendoza, 8, Fontana, Mango Elementary)
 
Google Groups: alt.humor
Halloween Humor - 18
Craig Bennett
10/15/98
(...)
Why do vampires drink blood?
Because coffee keeps them awake all day!
 
Google Groups: alt.brain.teasers
Halloween Riddle Jokes
Word Plays
10/30/98
(...)
Why do vampires drink blood?
Because coffee keeps them up all day.
 
Google Groups: rec.humor
JOKE MAIL
MCant23790
10/29/99
(...)
Why do vampires drink blood?
Because coffee keeps them awake all day!
 
Google Books
Best Halloween Jokes
By Melvin Berger
New York, NY: Scholastic Inc.
2001
Pg. 26:
Why do vampires drink blood?
Because soda makes them burp!
     
28 October 2003, Liverpool (UK) Echo, “Ghoulish giggles,” pg. 22:
Why do vampires drink blood?
Lemonade makes them burp.
   
13 January 2006, Herald Sun (Melbourne, Victoria), “Jokes,” pg. 72:
Q. Why do vampires drink blood?
A. Because coffee keeps them awake all day.
 
Twitter
Couch Sex Club
@CouchSexClub
Why do vampires drink blood? Because coffee keeps them awake all day! http://bit.ly/NMkb for @Astrogirl426
2:57 AM - 17 Nov 2008
   
Google Books
The Ultimate Everything Kids’ Gross Out Book:
Nasty and nauseating recipes, jokes, and activities
By Beth L Blair, Jennifer A Ericsson, Melinda Sell Frank, Colleen Sell and Aileen Weintraub
Avon, MA: Adams Media
2009
Pg. 8:
Why do vampires drink blood?
Grape juice makes them burp.
     
Google Books
Super Hot Jokes For Cool Kids!
By Various Authors
London, UK: Michael O’Mara Books Limited
2014
Pg. ?:
Why do vampires drink blood?
Because coffee keeps them awake all day.
 
Twitter
lorraine Jo King
@LahLahBean
#tuesdaytitters #halloween
Why do vampires drink blood? 
Because coffee keeps them awake all day!
🎃👻
4:20 AM - 30 Oct 2018