Is there a period when a person lands that he has vulnerability frames? Can he reversal (e.g: Dragon Punch) right when he lands to get out of this? Is a throw guaranteed when a person lands at a certain point?
Remember that ST (and HD Remix) are different from later capcom games in that throws have 0 frames of startup. This means that if you time it a right, a throw could theoretically beat out a 1 frame startup move.
If your character attacks in the air, he/she loses trip guard. The easiest way to test this out is to have one person jump and immediately do a hp or hk, (so that when they are descending form their jump they are not attacking) and have the other character play as dictator and do a sweep such that the jumping character lands on the sweep. You should find that when the jumping character lands he is unable to block the sweep, no matter what you do IF he lands on it. If the jumping character does NOT attack, I am tempted to say that he STILL does not have trip guard (i.e. will also be vulnerable to a sweep) but I am not sure. Are there vulnerability frames? Yes, in the sense that there are frames during which they cannot block, BUT…
To further complicate the answer, I believe that the lack of trip guard does not mean the character is “frozen” and cannot act; it just means that he cannot block. Thus, upon landing, the character could do any move (dragon punch, fire ball, command throw, s lk, etc). Can he reversal? Yes.
Lastly, ignoring the differences in throw range, there is no situation in which you can NORMAL throw someone in which they cannot NORMAL throw you. By this, I mean if two characters with equal throw ranges are trying to throw each other, there is no situation in which one character can throw but the other cannot. So to answer that part of your question, the answer is no.
I hope this answers your questions, but more importantly, I hope my answers are correct. 
if i slide you with bison as you land from an empty jump, do you think doing a srk will work and come out and beat the slide? But, if i reset you in the air with a non juggling normal, can you then do a reversal srk upon landing?
Does ST not have throw invunerability at certain junctures, like on wakeup?
Maybe they are, maybe they’re not, but if you’re not sure you shouldn’t answer. I’m no expert, but i think you’re wrong. hopefully someone can clarify.
No, a Shoryuken will not beat a slide after an attacking or non-attacking jump. Everytime you jump or land, you have jump start up and landing recovery frames. During the landing recovery frames, there is no trip guard, and the Shoryuken can only come out after the landing recovery frames are finished. So, if the slide or sweep hits during the landing recovery frames, the Shoryuken will not have a chance to be executed.
After being reset in the air, reversal Shoryuken will work, as those landing recovery frames are different than the regular jump landing frames. It would be the same if you were waking up or landing from a teched throw.
The only time you can’t be thrown is during knock-down, block stun and hit stun. The only time you can’t be air-thrown, is during air-reset (eg hit out of the air with a normal), teched throws or throws that reset you (eg Noogie, Knee Bash) or when you’re grounded (of course).
I’m no expert either, but even the experts can sometimes be wrong. That’s why ppl sometimes get a second opinion from multiple doctors, car mechanics, etc. I think that majority opinion should be the deciding factor. I agree with the answers that he posted, except for the part about reversal Shoryuken after landing. This is only possible when being reset out of the air, on wakeup or after hit stun or block stun.
You can definitely block on your way down if you are recovering from hit stun, or from teching a throw. Otherwise, Bison would get a free slide kick every time you teched his throw, which is clearly not the case.
You cannot be thrown for 13 frames after getting off the ground. This prevents Zangief from doing SPD after SPD on you for free.
Well, every character has a reversal to get out of throws, so that’s not what it really prevents.
my questions were rhetorical, since i was pretty sure i was right.
I’m pretty sure the “don’t attack in the air and you have tripguard” thing only applies to alpha 3.
Correct. ST has no tripguard. That was added in the Alpha series and games afterward.
follow up question:
do all jumping moves of a given characters’ do the same block stun? So if I do a jumping early hk vs a jumping early lp, will those do the same block stun?
Asking this question mainly in regards to tick throwing.
Absolutely not. Heavier attacks deal longer blockstun.
I’m always amazed at how many “safeguards” against abusable tactics were built into SF2. I mean, this was the very first real fighting game–the way we think of them today–so there was no precedence, no examples, no other titles (good or bad) to learn from or to help identify problems. It’s incredible that it turned out so well.
CvS2 also.
Tripguard is kind of a misnomer since you actually can’t even block standing attacks (Old Ken stand roundhouse comes to mind) for a couple of frames when you are landing. This is another reason why fireballs traps are so good in ST. I’m pretty sure you can however try a move with instant invulnerability like a shoryuken upon landing to get out of this kind of trap but that’s not entirely reliable since you’re just as likely to be taken right out of the air by most tripping anti-airs.
Sorry if I’m repeating anything that was said previously, I didn’t read the rest of the thread because I didn’t feel like it.