textual arguments must be enclosed in quotes
aa = DSum("IncTotal", "Income", "mmyy = '" & vmth & "' AND IncCA = & vt & " AND Incyy = 'vcy'")
... AND Incyy = 'vcy' ... :)
Hint:
The third argument of the Dsum() function (and similar ones) is constructed according to SQL WHERE rules
Good luck!