You can emulate a Theremin better using analog monophonic synths like Minimoog, MS20 or VSTi like Mercury than using samples. The important things are the legato and vibrato, wich are easier with analog synths... You can try a cabinet simulator (a cheap one) and a short slapback delay to finish the job.
The Theremin was invented in 1919 by a Russian Physician, Leon Theremin.
It\'s basically a box with two high frequency oscillators controlled by two antennas, one for volume/intensity control and another for the notes. The player uses his hands around the antennas.
A lot of film composers including Danny Elfman used it (Mars Attacks), it\'s the sci-fi retro sound that you can hear in every horror/scifi movie from the 40/50\'s...