I also made the 17th province I conquered a vassel and established trade with them. However, within 2 or 3 turns, they broke our alliance and declared war on me along with everybody else. However, I had read quite a bit about this over at the Total war forum and was pretty much prepared for it.

I had good size armies holding Bitchu and Izumo, which were the two only provinces at the narrowest part of the mainland, so any land attacker had to come through there.

I had 3 or 4 good size navies patrolling the coastlines and that is where I sunk at least four and maybe five invasion fleets carrying full stack armies.
None of them ever made landfall.

Of course my trade went to pot, but I had all the western trade nodes and had built up an economy to still support about 4 to 5k in koku per year.

I took my time conquering Shikoku island and then marched on Kyoto. It wasn't that bad. Of course nobody left would trade or make peace, so i just said---screw it, it's Total War; tell em', I'm a comin' and I'm bringin' hell with me. (with a nod to my favorite western: Tombstone)

Cheers