ok....
Well, why is he skinny in the first place? Did you buy him skinny, or has he got skinny in your tank?
If you bought him skinny, and otherwise feed your fish well, then I would let well alone and he'll fatten up nicely in no time.
If you bought him nice and fat and he's got skinny, then you are simply not feeding him enough.
French angels are probably one of the easiest to feed, so you shouldn't have a probelm.
As far as soaking in vitamins - something I've never done, personally..... its good if you disipline yourself to doing it.... but I do not.
To keep any angel fat and healthy they need lots of food..... quanitity is every bit as important as quality..... at least 50% of its diet should be nori - fed daily. There should be nori in the clip all the time so it can be grazed on at leisure. A good quality dreid food is essential in my view also, as it contains all the vitmains and nutrients that are needed. And finally, something meaty, like angel formula is good.... brine shrimp is pretty useless - mysis or plankton is MUCH better.
Here is whay my 6" emporer gets:
Lots of nori
3 different flake foods, mixed up.
Frozen food - consisting of:
ENRICHED brineshrimp / mysis / plankton / squid / mussel / angel formula.
Gernally, when I get in from work at 5pm they get flake, shortly followed by nori. I disolve a few cubes (any combo of the above) rinse, and then feed half about 7pm, and the rest at 8 or 9pm..... the nori is usually gone about 10pm......
My emporer is fat, healthy, and have grown from 3" to 6"+ in about 10 months, and is about 3/4 through his colour change..... by any standards that is a very good rate of growth..... so I must be doing something right.
The key, in my view, is striking the balance between feeding heavily without over feeding......