Use the same CultureInfo/NumberFormatInfo you used to format the number to that string format to Parse it back to numeric data. Here's an example using a custom NumberFormatInfo (because that formatting differs from my default culture number formatting):
You can see how using the the current culture info will allow you to format numbers like user would like to see them wherever in the world, and parsing it back again.
Dim nfi As New Globalization.NumberFormatInfo
nfi.NumberDecimalSeparator = "."
nfi.NumberGroupSeparator = "'"
Dim d As Decimal = 1000
Dim s As String = d.ToString("n2", nfi)
d = Decimal.Parse(s, nfi)