I think it would be easier to just use compost with a "good" mulch over it (or a compost/mulch mix) and let the worms do the "vertical" work. Mine do, after years of spreading compost on heavy clay we find the worms wintering over in the heavy clay several feet down. They're happy (and multiply) cause I feed them and I have less work.

I bet vertical mulching would work great.

I took a short landscaping class from a teacher who got the contract some time ago for landscaping a golf course in Houston where there is a lot of clay soil.

He said he did a lot of studying and homework and decided to do vertical mulching and he never looked back. He said it was tremendously successful and in his current business, he uses an augur to drill the holes and put in 1/3 volume of manure, 1/3 volume of vermiculite and 1/3 volume of sand.

I'm sure you could vary this with lots of different additives but if you've got clay, I would include some sand (rather than pea gravel) but I'm no expert. I also wonder if getting a lot of coffee grinds from Starbucks would be useful including that in the mix but then the coffee grinds might act like compost and compost will eventually be eaten by all the microbes, hence the need to constantly replenish compost. I would think compost would be better on top rather than in the vertical mulch, but again, I'm no expert.

FYI - He also advocated using bonemeal in any planting hole before adding the plant.

And he also advocated spraying your lawn with a soap and water mixure because it helps your lawn absorb the water better. (But don't use anti-bacterial soap). He had quite a business but was going into semi-retirement, hence the fact he was teaching the class.

Good luck and keep us posted.

"Vertical Mulching" is simply a not very good way to get organic matter into the soil, and the only reason tremendous results are seen by those that advocate it is because the soils they are working with are usually so bad that any organic matter would be an improvement. Just laying some organic matter on top of the soil will do the same thing, with much less work on your part. How quickly the OM will get into the soil will depend on the Soil Food Web that is there, ie. in a good, healthy soil the OM will be moved down faster (digested) than in a soil devoid of any OM. The only reason there may be for putting pea gravel in the holes drilled would be to improve drainage, and if drainage is bad enough to require that even more drastic action needs to be taken.
Very few horticulturists or soil scientists today will recommend putting bone meal in any planting hole, and they have not recommended that for at least 10 years now.