I'm not sure what exactly the vertical tee with a diagonal brace on it looks like, but I'm sure I wouldn't like it based on those words alone.

A couple thoughts:

1) It was probably added to fix an existing problem not considered in the basic analysis. You'd need to find out what problem was fixed before you determine which bad design is still the better design.

2) Maybe they're thrusting open the liquid valve abruptly and there's an impulse / thrust component they're considering - correctly or incorrectly. Or they're treating it as a hammer situation.

3) Looking at Danb's links, it appears the analysis is mostly about temperature, thus we can infer some similarities: too little liquid flow and lots of gas flow, and it rides one wall of the pipe. Too much liquid flow and too little gas flow and it'll ride the opposite wall of the pipe. Turbulence enhances mixing and temperature transfer, but more turbulence means more speed also means more pipe affected.

We can't really conclude anything about vibrations from the link since I don't think any of these diagrams are a gas/liquid isosurface (just TEMPERATURE isosurfaces). But I would expect that you could at least do a superficial flow regime / EIG check.