Working with titanium is difficult, especially when your have 6/4 titanium when it is brittle and really thinly shaped tubes on the Vortex.
Depending on your weight and riding, 6/4 ti Vortex may not be the right frame for you if normal riding stress is that great that caused the downtube to crack.
Move to a ti frame with 3/2.5 round shape tube, maybe slightly more weight on the frame, but sure likely to last longer.