Can you share your set/rep and type of exercise you find best?
My core is similar, very strong, but makes me appear blocky or square. I've tried some vacuums, but that really feels weird, especially when trying to eat frequently, are they truly effective?
First, off, I am so sorry you have the same issue. Its a drag and I feel your pain.
Regarding vacuums, I am certain they work although I have never used them as an exercise.
I actually still follow Arnold's guidance from his Encyclopedia. He lists vacuums as an exercise and explains the mechanics. There is a lot of knowledge in those old books. Between Arnold's books and Dr. Squat's (Fred Hatfield). When I was a teenager, I used to go to the bookstore on the weekend and just stand there are read up on everything and learn. As a youngster I just could not afford to buy all the books. I did own Arnold's Encyclopedia though.
Ab routines...
During the off season (which is now forever), my ab routine is just a abhorrent amount of repetitions, vertical leg lifts, crunches, and twists with a stick. I do 100 leg lifts, 200 crunches and 200 twists (100 each side with the abs flexed the whole time) every cycle (5 days). I am sure my body would take more frequency but that is sufficient for me- I am not trying to EVER build mass in the ab area. I push myself pretty hard in that the pauses-when I am burning and have to stop-are just brief enough to dull the pain so I can continue. That is a little modded from what Arnold recommended but I am not him. LOL.
Contest prep, he has a whole different routine with lots of different exercises. For making my abs, obliques, etc, the bomb, I could not find better.
I am not sure that will help too much. I really would encourage anyone with our 'blockiness' issue to pick up Arnold's book, read it, and figure out what works best for you. I really do wish you luck though. If you come across something earth shattering, please share it.