I'm going off on a tangent here, so bear with me. Start by making sure you have your mix position symmetrically placed side to side and that your ~equilateral triangle is lined up right. Now get a baseline set of measurements for the room. Search out threads here about REW. Read this too: Before posting your measurement results
For room treatment in general, such as corner traps, just get some of the 6" thick unfaced attic insulation - fluffy pink insulation - and use that. You'll save $ compared to 703. I'd recommend doing a first level optimization of your mix position acoustics, then worry about the vocal recording treatments next.
So treat the front corners - and avoid the heat register on the right - with superchunks, put first reflection treatments behind your monitors and on the sidewall mirror points, then make a minimal cloud to treat the ceiling first reflection points. OK, now re-measure. Now go superchunk the back corners and measure again. What are those measurements telling you? How do you perceive the changes at the mix position?
For a vocal "booth", I'd make six gobos 24" wide x 78" tall filled with 6" thick pink fluffy insulation and put slats at ~50% coverage on one side and over 75% on the other. You could also go with 4" 703 on these if you want to. Put a 4x4' ceiling treatment above the vocal position and arrange the gobos Stonehenge-like around the mic and try different spacings and orientation. Singing is so weird for many people, to the point that if you can set up a comfortable space that has decent acoustics, the comfort part overshadows the rest of the equation. When you are not recording vocals, I'd use the gobos as a pseudo back wall with symmetrical air gaps.
If you want to go beyond the above, you could look at treatments such as polycylindrical diffusers or going after the remaining major modal issues with tuned pressure based traps.