Re: How can I get my alto to sound better?
Having played clarinet should actually work to your advantage. The clarinet requires more precise embouchure control than alto sax, and it is actually more difficult to play a clarinet in tune than a sax. You should hold the saxophone in the position that is most comfortable for you. I'm a pretty big guy; 6'2" and about 255 pounds. When I play alto, I hold the sax on an angle, the bell on my right side, and I turn the mouthpiece on the neck so that it is straight up and down. This seems to be the most comfortable position for me. You may need to relax your jaw a little more. Saxophone doesn't require as stiff an embouchure as clarinet. A good neckstrap like the Neotech might help also.
I would try a La Voz medium reed instead of the Vandoren. I think you'll find that they are much more consistant, and they play better. The Rico Jazz Select reeds are pretty good too.
Another suggestion: for about $17, you can buy a Rico Royal Graftonite mouthpiece. They're cheap, but they sound really good. It would give you a bit more edge than the yamaha mouthpiece.
But, the main thing is practice and just getting comfortable with the instrument.