A person who plays the Portables! I like you!
It's alright that you play with Vita. I started out playing on the PSP. The arrow controls make it easier to input IMO, so if you like it that way, that's okay.
The main difference between the real competetion is that they play consoles. Be warned, because on console, you move more freely than you do on the portable games. That's what spoils you. You may have some adjusting to do when playing on consoles. Also, I prefer the four arrow buttons as opposed to a joystick. It seems to give one less control that rolling your thumb around.
1. Picking your main should be of ease to you. I tried all the characters, and found Tsubaki was comfortable at a beginner's pace, and got gradually better when getting more competetive. Jin should be a the first stepping stone; he has projectiles and a vary assort of special (and using 25 heat to upgrade them like in most games). He has a decent reach with his normals too, so try him and see what you like. If you like using the projectiles, choose a zoning character like Nu or Rachel. If you like using the sword a lot, you can go with Ragna or stick with Jin. If you like punishing characters (like using Jin's counter special), try Hakumen. If you want to be faster, try Tao. If you like closer action, go for Makoto or Tsubaki. But it's up to you.

2. Being a portable player limits you, and isn't really regarded highly. But I mainly play portable, so that's fine. I find it easier that you can stare down at the screen
and at the controls at the same time. Hopefully PS Vita can play the PS3 in CP, because it will make it portable friendly. (that, and I want some salt when a player uses portable and wins)
3. A lot of practice (That darn P word pops up). PSP and Vita are fairly simple to grasp (especially PSP). But Xbox controllers and joystick are harder to use. Try getting comfortable with the portable controls, and then move foward.
4. and 5. are already answered.
Good luck!
And no, I totally haven't been waiting for a portable user to ask for help 