First of all create all variables in analyze sheet..even your vFileOld and vFileYest should be made in analyze sheet and both should be blank variables with no definition.

Then create another two variables which will select your files as per your requirement..

For example vFileOld1 = if('$(vFileOldSelectedAssigned)'<>'',

'$(vFileOldSelectedAssigned)',
if(day(Today())=1 or day(Today())=2,date(AddMonths(today()-day(today()),-1),'YY.MM.DD'),date(today()-day(today()),'YY.MM.DD'))&'-SHOO.qvd')

Similarly create vFileYest1..

Now in automation use Get Expression value blocks and give inputs as =vFileOld1 and =vFileYest1

Now using Update variables block update variable vFileOld and give its definition as = Get expression value1..

Similarly do it for vFileYest also..and then use save App block and then do reload..it should work smoothly..its working smoothly in my case..