You can try with the following WHERE clause for TAB1 (add quotes)
from Table1 where Date > '$(vMaxDate)';
First time around, this code will load all rows. If that's not what you want, set the default value for vMaxDate to some exceptionally high date value that none of the available Dates will match.
May be even if values in vmaxDate there or not and you want to run table 2 then You can try like this:
Here not adding anything btn else and end if bcs we don't want it to do anything If vmax Date id null..Plss Try and let me know.Hope it helps
If not isnull($(vMaxDate)) then
Load from table 1
Load from.table 2