=SUMPRODUCT((B1+1)^ROW(INDIRECT("1:"&B2)))

Public Function vini(x As Long, N As Long)
    Dim i As Long
    For i = 1 To N
        vini = vini + (x + 1) ^ i
    Next i
End Function

Public Function summation(lower As Long, upper As Long, summand As String)
    Dim i As Long
    Dim total As Long
    Dim original As String
    Dim temporal As String
    total = 0
    For i = lower To upper
        temporal = Replace(summand, "i", i)
        total = total + Evaluate(temporal)
    Next i
    summation = total
End Function

=summation(lower bound, upper bound, summand)