I think you're pretty much there, you just need dollar-expansion around the variable:
LET vCutoffDate = makedate(2013,10,30);
LOAD [Invoice Date], [Standard Invoice Qty], [Invoice Amount], [Cost Value], [Invoice #]
Sales.QVD (qvd) WHERE [Invoice Date] >= '$(vCutoffDate)' ;
However, using the WHERE clause here will break the optimised QVD load. You may want to think about pre-loading all possible dates greater than your variable, then using WHERE EXISTS([Invoice Date]) to keep the load optimised.
Hope this helps,