Total healing on average in combat would be:
Knights Vow-
If adc deals 500 damage per AA/spell
500/1 x 6/100 = 30 (+38%)
= 30(+11.4) = 41.4 HP healed per second.
In addition to that:
Rod of ages heals 25 hp per spell cast
25(+38%) = 25 + 9.5 = 34.5 HP healed per spell cast.

41.4 + 34.5 = 72.9 HP healed around every 1.5 seconds in combat on average (more if adc hits around 600 - 1200 dmg per basic attack)
The infinite Mana works with knight's vow. 6% of damage dealt to the adc is dealt to you instead, which converts 15% of that 6% to mana. In addition if they attack you, you regain 15% of their damage as mana.

This build gives soraka a tanky touch, so it will be hard to kill you plus you give your ADC constant heals while they heal you, you regain your HP spent to heal in combat thanks to the health regen from spirit visage and mikael's crucible, in addition to the heals from combat stated and explained in the above chapter 'Math behind this build'.