In a stack, all the stack members act as one switch, so when you configure a standard LACP LAG that spans all stack members, you are aggregating the links across the stack. In other words, a standard LAG will work across all members of a stack whether backplane stacking or using VSF.
This would be an example of a LAG over different stack members:
trunk 1/A1,2/A1 trk1 lacp
Justin