This expression will expect Date to also have the format 'DD MMM', and both [Date] and the variables must be numerical date values, not text, for the range comparison to work correctly. I assume that DD MMM is not the default date format for you system.
vMaxDate should be a date value, but vMinDate will be a text value. Rather define it like this:
Let vMindate = Date(Date#('3 sep' & Year(Today()), 'DD MMMyyyy'), 'DD MMM');
and make sure that Date is a numeric date value.