You shouldn't instantly air dodge after the throw you're going to get hit by vanish and uair that way. You have to wait a little and then air dodge. Like I said before sheik shouldn't be able to kill you at certain percentages so there's no real reason to air dodge right after the move.
Like you've stated side B will only allow you to avoid the vanish it won't allow you to cover the uair. The vanish is easier to avoid than the uair. So you should probably treat the sheik like she's going to uair you. I sti say drift+ air dodge is your best bet.
Ok, lets start small.
http://smashboards.com/threads/link...-found-in-the-op.379659/page-33#post-20022628
Depending on how hard you are hit (how high your % is) you can Air Dodge sooner than you can do any other action. Through testing it has been said that you can Air Dodge 4/6 frames before anything else depending on knockback and hitstun.
This is fact #1.
Fact #2: Sheik's Throw to U-Air starts as a true combo, completely unavoidable, nothing you can do to stop it. We can all agree on this right? As such it is quite obvious that Sheik has + frames on her Throw that are greater than her endlag + jump squat + how long she takes to reach you + U-Air startup
Fact #3: As you get higher % the Throw has higher knockback which makes Sheik last longer reaching your position. You get put into more hitstun too, but the frames she requires to reach you are higher than the extra frames of hitlag.
Fact #4: There will be a % where Sheik will not be able to reach high enough before you can just jump out of the combo.
Ok, using math we can deduce the following:
In between 2 and 4 there WILL DEFINITELY be a % at which U-Air will hit you exactly 4 frames before you can Jump. Since you can Air Dodge 4 frames earlier then that means that if you were to Air Dodge at this % then you would avoid the U-Air, but if you were to do ANYTHING else then the U-Air would hit you.
What if Sheik reads the Air Dodge? She can wait it out and U-Air you right as you come out of the Air Dodge, if she guessed it and timed it correctly this is unavoidable. THIS is what people refer to as the 50/50.
Sheik also has a second option which is a super early kill move, which is going for Vanish. YOU are claiming that you can just wait and air dodge afterwards. We ALREADY STABLISHED that Air Dodge only comes out 4-6 frames before jumping, since U-Air definitely comes out before you can input a jump this means that the MAX amount of frames that you "can wait" is 3 frames otherwise the U-Air would hit you.
EVEN if you had inhuman precision and were able to wait exactly 2-3 frames (which I don't think it's possible for you to measure AT ALL). Vanish has 3 active frames, so if you have this inhuman precision, we should assume Sheik also has this inhuman precision and is able to time Vanish to overlap perfectly with the 3 frames that you "can wait".
At the end of the day, it's still a 50/50. Sheik just guesses whether you will wait 4+ frames or whether you will just air dodge immediately. If you waited and air dodged then you could have jumped out (well maybe not Mewtwo since Mewtwo's jump is ass. But other characters could have) which is a much better option.
There's a certain % at which you will just be able to jump everytime too, but at that point it's not considered a 50/50 anymore. It's also not considered a 50/50 when it's a true combo either. There are also certain characters who might get options available to them depending on their weight/fall speed and their aerials/specials. Like Mewtwo who can Side-B at some %s, but unfortunately. if the Sheik reads you then Side-B is just as risky as waiting is.