So we have the string below as:
VAN909_445_Viperacrfd_40450.jpg
Assuming that VAN909 is always the same at the beginning and if you can get the id, can you not do something like:
string actualText = "VAN909_445_Viperacrfd_40450.jpg";
string id = 445;
string textToRemove = "VAN909_" + id + "_";
string newText = actualText.Replace(textToRemove, "");
That should do it if the assumptions were used.