I generally sail with a seat harness but Ive been trying a demo mystic waist harness.The waist is certainly easier to hook in & out of & sail in the classic 7 stance.All the info in forums etc seem to be aligned to suit waist harnesses.
Anyway..now I know why noone else seems to use a buoyancy vest @#$ [}:)].I couldnt find anyway to tie the stupid thing down & whenever I fell in it was up around my ears & made waterstarts & everything almost impossible.I ended up coming in & switching back to the seat harness with slimline vest that I have clipped down onto it.
With the waist harness I was using a yellow zip up boating vest which was shorter but more volumous.I couldnt use my slimline ski vest as it was too long & would go down over the hook. I use a vest to aid waterstarting + its a legal requirement here although I dont know how often the MSB come around.

Certainly looks more slimline than the bulky vests I have!Once the gear is dry from yesterdays session Ill try the waist harness on over the slimline vest or try the other vest+ see if i can rig a strap under the hook .

my bouyancy vest is on under my wetsuit skin

I dont think the NP vest takes a hydration pack.

They are definitely a great thing. Wouldnt go out without mine on - makes water starts easier and if something was to go wrong you save a lot of energy with the added buoyancy

if ya chuck a spray jacket or a rash shirt over the top of ya jacket and then put ya harness on .. the elastic on bottom of jacket or rashy hold ya vest down and in close to ya body tight

The other option SBoardcrazy is to buy a Neil Pryde impact vest. Its a chest harness with a higher hook than a seat harness. It gives good floatation, has a quick release system incase of getting tangled up with the harness lines. It gives support to your back and midriff.

The only real minuses are the price and the fact it doesn't have a little pocket to put your keys like the old Da Kine harnesses do.