Sorry, a point of clarification: the issue is with the advanced variable creation (not the data variables - these can't change as they are used elsewhere). Line 3, below, is my latest try - lines 5 and 6, below, are my prior tries.
//Second Try, does not report the specific row error
LET vFrom = IF($(vMonthStart)<'11/01/2015',$(vFromOld),IF($(vMonthStart)>'10/30/2015',$(vFromNew)));
//First try is below (both rows) - says error at","
// IF($(vMonthStart)<"11/01/2015", LET vFrom = $(vFromOld));
// IF($(vMonthStart)>"10/30/2015", LET vFrom = $(vFromNew));