It's important to understand that benzos are a class of medication, not an active ingredient. Not all medications of the same class have the same effects, and in some cases they do not even have the same method of action. Meaning they are not always a clear substitution for each other. Further, if they DO substitute for eachother the dosage of one to be the equivalent of the other may be wildly different. Vallium is not equivalent to Xanax. The dosages are different, the side effects are different, and while there is some overlap in what they can treat they both act on the body in different manners and can treat different conditions and have different withdrawal symptoms.
Now, back to your active ingredient question. For instance we will take vallium, it is the same active ingredient as Diastat AcuDial, Diazepam Intensol, and Diastat. These are 1:1 substitution drugs and brand name only changes the color and preservatives they are manufactured with. You'll note Xanax is not on this list as they do not share an active ingredient.
Will vallium work in place of Xanax for the method you're considering? I don't know. I'd carefully research the PPH and threads here before making a decision.