"For matters majestic,
the King elects
To neglect the domestic."
When Don King asked Benjamin to make a gem for a sizable loan to Slately, Benjamin actually refused. He justifiably claimed they couldn't afford it, which is exactly what Don King told Slately when he was first asked. Benjamin went further, saying he needed to hear what Caesar had to say before he could loan desperately needed shmuckers to a shaky side, and eventually (respectfully) refused the order with the unanimous backing of the half-dozen warlords at the capital.
Benjamin acts as an advisor to Don King; possibly like a consigliere or finance manager.
Benjamin is apparently not a Transylvito Vampire.
Ben appears again in LIAB Text 34 reciting (or perhaps only thinking) a piece of Rhyme-o-Mancy in which he criticizes Don King for spending schmuckers on Jillian and Faq. Since he is 'cancelling the antsy' with his rant, it may be that Benjamin is the only thing preventing Caesar Borgata and the warlords from carrying out a coup, or perhaps he's merely calming himself.
Perhaps Benjamin is similar to Wanda in being proficient in many magical disciplines outside his own, but unlike Wanda (and more like Sizemore) actually finds them fascinating. In the real world, Ben Franklin was a brilliant polymath: a scientist famed for his work with electricity; author, publisher, copy-editor, and printer of Poor Richard's Almanack, including assorted epigrams, minor poetry, and weather forecasts in it; a mathematician who explored "magic squares"; and one of the major political leaders of the American Revolution against King George III.
Real World References
Benjamin's name and appearance is likely a reference to Benjamin Franklin, especially his depiction on the one hundred US dollar note.
"Benjamin" is also a slang term for an one hundred dollar bill; "Benjamins" can be used to refer to large sums of money.(On-Line Slang Dictonary)