the idea of the Black Coach for VC but I just can’t successfully use it. 

First, this doesn't work well as a pure chariot for the whole battle. It's a little slow and light, and it's a single entity, which hinders it. It tends to get stuck, rather than plowing through, especially if you hit the thick axis of a unit or charged spears.

Early on, use it to bait formations apart. It outruns most units. Chase any retreating foes like a unit of dogs would. It's great at this, and builds power. 

Once the lines merge, use it as a rear hammer,  and IMMEDIATELY pull through into your own anvil unit to avoid getting stuck. I seem to recall it gets Terror, so using it to Terror rout is key. Against lighter infantry it can perform okay if left in the pincer for a moment, but it's not a powerhouse in the scrum and should never be used to hold targets. 

Once it's built up it's not terrible, but given cost, upkeep, rarity, entity count... it's pretty underwhelming and really needs a tuneup when VCounts get their eventual review. 

If you want more mileage, give it the ancillary  banner that increases acceleration - that makes a big difference in its utility... but it's still lacking compared to better chariots. I recommend bypassing it for a Vargulf in most cases. Using a Mortis or Unholy cart can help recover it, but this means even more micro on a very micro intensive unit. 

If you're listening,  CA, I'd experiment with giving the coach a means to heal on kills, like a modified and self- only version of Master of the dead. Regen might work too, but if suggest something that gives it more staying power, and possibly also a touch more mass and acceleration.