I don't know (I should know) the back story on JB's preference of VM's. At one of our gigs, I asked a friend what he wanted and he said "a martini". I got him a vodka martini and after sipping he said "what is this?" (expecting gin) I'm not really a martini person so I only thought of JB.
I follow the Winston Churchill ethos of martini’s.
"Glance at the vermouth bottle briefly while pouring the juniper distillate freely."
for me. 

