You mean Red Focus?
Vega can go for that after a standing Hk or the last hit on Rolling Crystal Flash.
With that said, the best one (or at least, the one i get to use the most) is the one i wrote before.
Cross up j.Lk> cl.Mk > st.Hk xx Red Focus > Ultra
There is also
st.Lk> cr.Lp > st.Hk xx Red Focus > Ultra
Keep in mind a few things.
St.Hk is 4 frames start up
St.lk is +5, so you can link those two without the middle cr.Lp, but you’ll have less time to hit confirm.
You can only use Red Focus on the “first hit” of his st.Hk
Basically, anything that can be linked into a st.Hk can lead to RF, with the only problem being spacing.
So, for example you can do cr.Lp> cr.Lp> st.HK xx RedFocus