Two options:
VLOOKUP
function in its own cell: =VLOOKUP(A1, B:B, 1, 0)
(in say, C1), then formula referencing C1: =IF( C1 > 10, C1 - 10, C1 )
Function MyFunc(a1, a2, a3, a4)
Dim v as Variant
v = Application.WorksheetFunction.VLookup(a1, a2, a3, a4)
If v > 10 Then
MyFunc = v - 10
Else
MyFunc = v
End If
End Function