SSRS Last Week Monday and SUnday

To get Sunday from Last Week : (Note: Last Week Sunday is different from Last Sunday).

=DateAdd(DateInterval.Day, 1-WeekDay(Today), DateAdd(DateInterval.Day, -7, Today))

To get Saturday from Last Week :

=DateAdd(DateInterval.Day, 7-WeekDay(Today), DateAdd(DateInterval.Day, -7, Today))

with Weekdays values as below:
1 - Sunday
2 - Monday
3 - Tuesday
4 - Wednesday
5 - Thursday
6 - Friday
7 - Saturday

SSRS VBA allows very similar expressions for date manipulation to Sql, the main difference being the use of the DateInterval enum. Without checking your expressions, it will convert to SSRS VBA as follows:

Last Monday:

=DateAdd(DateInterval.Day, 2-WeekDay(Today), DateAdd(DateInterval.Day, -7, Today))

Last Sunday:

=DateAdd(DateInterval.Day, 1-WeekDay(Today), Today())