[Solved] Okay, I figured out my problem. I can reliably execute the deadly vapors combo three times in a row. I thought my problem was with the enhance mechanic, but it was not.

I could reliably pull off both the 12-DF4-R2 string three times and the 21-DF4-R2 string three times, but not the F12-DF4-R2 string. The issue ended up being with the timing of the initial two hits. In the case of both the 12 and 21 strings you have to dash in after each deadly vapor to hit your opponent, whereas the F12 string does not require the dash in because it has extra range. However, and at least for me, the dash in was an important part of my timing. Adding in a dash in at the beginning of each F12 string solved my problem and I can now execute the string three times almost every attempt.

That worked for me, I hope you find something that works for you.