I doubt either of the two statements work. The apostrophes are badly placed. Should be:
aa = DSum("[IncTotal]", "Income", "[mmyy] = '" & vmth & "' AND [IncCA] = " & vt)
and
aa = DSum("[IncTotal]", "Income", "[mmyy] = '" & vmth & "' AND [IncCA] = " & vt & " AND [Incyy] = " & vcy)
Cheers,
Jiri