Are there any elements in common between the VAR* ? If there are, then saying "which" variable the common element came from is not meaningful.
If there are not, then create a lookup table such as
{ repmat('VAR1', 1, length(VAR1));
repmat('VAR2', 1, length(VAR2)) }
and then in the nchoosek call, for VAR1 substitute 1:length(VAR1), for VAR2 substitute length(VAR1)+1 : length(VAR1)+length(VAR2) and so on, so that you are sending in integers 1 : length(everything together). Then for each nchoosek result, you can index the lookup table at each element in order to find out which variable name the element originated from.