配列のインデックスを関数で返せばいいじゃん。

Public Sub Main()
Dim a(1) As Long
Dim c As Boolean
Dim i As Integer

i = f(c)
a(i) = a(i) - 10

End Sub

Public Function f(c As Boolean)

If c Then
f = 0
Else
f = 1
End If

End Function