If you mean to replace the content of each line before
:
then the following code can be used
Dim infos As String() = System.IO.File.ReadAllLines("TextFile.vcf")
Dim modifiedInfos As String() = infos.[Select](Function(ln) _
If(ln.Contains(":"C), ln.Substring(ln.IndexOf(":"C) + 1), ln)).ToArray()
System.IO.File.WriteAllLines("modTextFile.vcf", modifiedInfos)