I'm not sure who told you that, but putting a bass string on a guitar will probably really screw up the guitar. For it to be in tune he'd have to have the tension on the low E (assuming he used a bass G string) so high that the neck would likely warp. Additionally, I'm not sure the ball end would even fit through the bridge, but I guess that would depend on the brand. Even though the gauges are similar, they are different strings.
At any rate, I'm pretty sure Vaughn used D'addario .013's with a .054 gauge low E.
Edit: I was wrong about his brand and E gauge:
http://www.jcdisciples.org/musicians/srv/g...itar_setup.html
Stevie tunes his guitar down a half-step and uses GHS Nickel Rockers measuring .013, .015, .019 (plain), .028, .038, and .058. On this particular day, Rene had substituted an .011 for the high E to keep down the sore fingers that blues bends can cause. Rene changes strings every show for each guitar that gets played.
(Rene Martinez was Stevie's guitar tech)