You can execute a VB Script function using the statement

ExecuteFile "File"

Eg: If you have the .vbs (MyVBScript.vbs) file in the 
path "C:\Temp" then the following statement will call the 
same:

ExecuteFile "C:\Temp\MyVBScript.vbs"

to call function in QTP go to 
test--->settings-->resorces-->add the .vbs file

correct me if am wrong.

Above two answers are correct but they forgot to explain
about the differences of those?

1. Call .vbs file

Test Settings-->Resources-->+ button-->show the path of .vbs
file-->Click OK button

Click the "Check Syntax button"

It is available for all actions in the test. If you click
"Set as Default" It will be available for all the new Tests.

In Test Pane.

Call functionname(parameters)
or
var=functionname(parameters)
msgbox var

2. Executefile "path of .vbs" file

It is used to execute the function dynamically but it is
used for only particular action,