Victaulic families are, I think, designed to work with their own Victaulic Tools for Revit plug-in, so I’ve tended to find that when used in native Revit they might need a bit of modification.
For what you are trying to do you would need to edit the family and I think it would be something like:
- change the part type to Tee
- add an extrusion along the main axis, to simulate a piece of pipe
- align and lock the ends of the extrusion to two reference planes. They should be equally spaced either side of the centre left/right plane and the distance between them controlled by whichever parameter controls the overall length of the fitting.
- the extrusion inside and outside diameter should be controlled by parameters matching the pipe inside and outside diameters in the pipe segments & sizes settings (you might need to modify and reload the lookup table and also maybe adjust formulas)
- add Fitting connectors at each end of the extrusion and associate their radius with the nominal radius parameter
- One of the original connectors can/should probably be removed (not the one which controls the branch size)
- delete and purge the original family out of your project file
- load the modified family.
It should now behave as a tee and adjust to the size of the main pipe (hopefully).
What version of Revit are you using?