The fact that you're having problems with precisely every character that does the dragon punch makes me think you are jumping into them to much, or hovering overtop of them when you knock them down. Don't try to dropkick into the character unless you time it perfectly, and always backup once you knock them down.
I just tried 7* last night after reading this thread and got to Vega so some comments against Ryu and Ken (both using Ryu).
Ken:
- He is extremely aggressive, and both the hurricane kick and dragon punch leave him vulnerable. This is a fairly easy fight to win. Just let him attack you, defend it, then counter with a throw. The easier counter with out having perfect timing is blocking the dragon punch then using the hard kick to do an axe kick while he's in the air. This will knock him back and you can combo it with a dragon punch before he lands if you're fast.
- He is also goes into a mode where he jumps back and forth doing drop kicks. Just let him come into you and uppercut him. I usually avoid trying to dragon punch to counter because if you miss it you'll end up fireballing and are vulnerable. If you have a perfect dragon punch then use it whenever he jumps into you.
- He'll do the fireball/fireball/fireball thing like Ryu but not as often. The trick to these is not jumping forward (unless you have a lot of space). Just block or jump over them, or counter them with your own fireballs if you're confident. After he misses a couple times he'll start moving forward against you again. Ken is too fast, so I don't bother trying to counter him (drop kick) during the fireballs.
Ryu:
- He is much more defensive then Ken and a pain in the ass! It's harder to get him to stop fireballing and come after you so you'll need to attack during his fireballs. The trick is getting to just the right distance from him that you can start your jump when he starts a fireball and you hit before he can dragon punch or uppercut to counter. Follow up every drop kick with a sweep.
- Use the same countering as against Ken if you can entice him into hurricance kicks or dragon punches.
- He will go into a mode where he drop kicks / sweeps you, so you need to do a hi then low defense or you'll get creamed by this.
Against either character just use lots of fireballs to try and keep them pinned once you have an advantage. Beat them at their own game. If you dodge enough fireballs they will come after you.