Summary:

Damage block is being calculated AFTER the 50/40/30% damage reduction, and is therefore causing windrunner to do substantially less damage to enemies with vanguard/PMS.

Repro:

1) Pick windrunner, level to 6 spawn an enemy hero and give them a vanguard. For ease of calculations I recommend a level 1 wisp (0 armour exactly)
2) Cast focus fire and fire away
3) Review the combat log

Result:

[00:43.26] Windrunner receives Focus Fire buff from Windrunner.
[00:43.86] Windrunner hits Wisp for 9 damage (723->714).
[00:44.19] Windrunner hits Wisp for 31 damage (716->685).
[00:44.49] Windrunner hits Wisp for 11 damage (687->676).
[00:44.79] Windrunner hits Wisp for 10 damage (677->667).
[00:45.09] Windrunner hits Wisp for 34 damage (669->635).
[00:45.39] Windrunner hits Wisp for 8 damage (636->628).
[00:45.69] Windrunner hits Wisp for 8 damage (630->622).
[00:45.99] Windrunner hits Wisp for 12 damage (623->611).
[00:46.29] Windrunner hits Wisp for 34 damage (613->579).
[00:46.59] Windrunner hits Wisp for 31 damage (581->550).

A 20 damage difference is observed...

Expected result:

Focus fire worked as a pre-heal in WC3 DotA, and therefore it calculated the damage you were going to deal and healed the enemy for half of that just before getting hit. So you should deal (for example) (60 - 20) / 2 = 20 damage minimum. Instead it's calculating (60)/2 - 20 = 10 damage.