I can't really give you tips on it. It's mostly a buffering thing. What I would do is do a combo like st.C, df+C, qcb+A, into qcfx2 +A
During the df+C, do qcb+A and before you see the motion come out for vapor cannon, just go in on the DM. The reason is that you have to do the motion on the impact of Vapor Cannon, not before or after. Use the long animation of df+C to get it in as fast as you can.
So the trick is really after df+C, do qcb+A as fast as you can and go into the qcfX2+A without pausing
The window is pretty small, but once you got it, you got it!
If you can't do that, what I would do is the qcb+A but drag along the sides into a half circle forward, then do a qcf+A. It's a little sloppy, but it guarantees you don't have to go back to neutral (Letting go of the stick or letting it go back to normal) to do the qcfX2 +A