There are (at least) two ways to do this. One requires that you write your "main" script as a Windows Script File (.WSF) instead of a naked VBScript file (.VBS), while the other does not.
You should look into WSH's .WSF file format, it offers many advantages, this one among them.
The examples here presume you want to make a logon script. Though they aren't practical logon scripts because they use MsgBox and do silly things, they illustrate the point.
Sub Announcement() MsgBox _ "Welcome to Veeblefester Widgets, Inc. logon script." & vbNewline _ & vbNewLine _ & "We hope you'll have a pleasant logon experience." & vbNewLine _ & "Be sure and stop by again soon for the finest logon" & vbNewLine _ & "you'll encounter anywhere!", _ vbOKOnly, _ cstrHead End Sub
Function Magic(lngX) Magic = (lngX + 34) / 12 End Function
As I said, you probably won't want to use MsgBox calls in a logon script, but you get the idea.
The first approach uses a .WSF script. These run with CScript.exe as well as WScript.exe just like a .VBS, .VBE, .JS, etc. It uses the src attribute of the <script> element to perform the include.
The second approach relies on reading the include file via the FSO and using the VBScript Execute statement. You might want to look into the limitations and possible side-effects associated with Execute before using it.