Jimmie Vaughan fast modulation effect

Hi - I'm hoping one of the gurus here can tell me what is the fast
modulation effect as used by Jimmie Vaughan back in the Fabulous
Thunderbirds days - I'm particularly thinking of the track One's Too
Many and A Hundred Ain't Enough - though several others, notably the
Slim Harpo tracks. It's a fast modulation effect - but not quite
tremolo, not quite vibrato (though I may be wrong here!) not quite
vibe, not quite Leslie (though I'm not sure here, given things like
Danelectro's Rocky Road) - aaaarrrggghhhhh! It's a killer sound - can
anybody tell me what he was using?

Thanks from London
~JB