[yuujirou]

Eh.... =\

Your problem isn't the rice...
It's not because you put in too much vinegar, and it's not because you too much water, and it's not because the rice is hot.

If you add vinegar to the rice while it's still hot, some of the vinegar will evaporate; the only way you've added too much is if your rice is literally sitting in it.

If you cook the rice w/ too little water, then the rice is going to be too hard and terrible to eat. Too much water and your rice will turn to mush. Regardless, rice absorbs a lot of water for it's size. So there's going to be a 'lot' of water in it even if you cook it right.

The temperature of the rice does affect the nori. However, sushi is meant to be served while the neta [toppings] are cold and the shari [rice] is warm [a little above room temp]; cold sushi rice becomes hard and difficult to mold as well as being less tasteful.

Your problem is your seaweed. The seaweed you buy at your local market is just really cheap and will get soggy within 2-3 minutes. Really nothing you can do about it. If I remember right, a 10ct. box of Yuuka grade nori from Yamasho costs about $500. It's one of the best grades of nori you can buy in the us.

And the shape of your sushi? The problem there lies in two factors. Firstly, I'm assuming you're using a roller with very thin and rounded bamboo strips. This makes it kinda hard to shape it 'perfectly'. Secondly, you're not doing it right. While rolling the maki, press down on the middle of the roll to form a square shape. Maki-mono shouldn't be a perfect circle.

The reason why your maki are falling apart is because a) you're adding too many toppings or b) you're using too much rice or c) you're just not rolling it right.
To roll it correctly: when you're rolling the seaweed over your toppings, make sure the seaweed side connects with the rice and that you pull the seaweed towards you to further compact the fillings.
Nori sticks to rice; but not to nori.

As for cutting the roll, you don't necessarily need a razor sharp knife.
Just make sure you're coating your knife in a bit of water to prevent the rice from sticking and it should be all right; although, make sure your knife can at least cut through butter. Otherwise, you may as well try using a mallet to cut it.

Also, your filling; make sure they're as dry as possible. Water causes rice to separate.

Hope this helps.