Basically three separate problems.

(1) Sheet1!A1:B3 Is not a valid thing to put in a function  call parameter.
It's going to need quotes around it.

(2) The second parameter actually needs to be a range reference, so you'll
need something like a Range call to convert it.

(3) You need to prefix Excel functions with WorksheetFunction to tell Excel
that they're not local functions.

I.e.

myValue = WorksheetFunction.VLookup(0.6, Range("Sheet1!A1:B3"), 2, False)