How many voltage points are we talking about?
My first thought is to try sending the list to the SMU separately as new array, breaking it up if needed. In pseudocode:
writeline(K2412B, strjoin({'vlist=',VliststrPart1)});
writeline(K2412B, strjoin({'vlist=vlist..',VliststrPart2)});
writeline(K2612B, strjoin({gate_smu,'.trigger.source.listv(vlist)'}));
(".." is the concatenation function in TSP/Lua.)