As I understand the question, you want to know how to combine the sentences given in 1, using syntax similar to 2, and understand what rule permits the combination.
The example you give is very nearly correct; this sentence:
Tet, which occurs in late January and early February, is a festival.
has the same meaning as given in 1. The dependant clause ("which occurs in late january and early february")can be added to the initial sentence ("Tet is a festival"), as it has the same subject. It has the full meaning of the first sentence (tet is a festival) and provides unrelated information (Tet occurs in late January and early February). When adding who/which clauses like this, the commas tell us whether the clause is specifying or adding information; a who-which clause with commas states information which does not change the meaning of the sentence; Tet is still a festival in all cases. If we added the clause without commas, as follows:
Tet which occurs in late January and early February is a festival.
The which clause would be specifying; this sentence states that Tet is a festival when it occurs in late January and early February, with the implication that Tet sometimes occurs at other times, and is then not a festival. The same is true of the other pair of sentences; this sentence:
Mrs Brown who is standing over there is my aunt.
States that Mrs. Brown is only my aunt while standing over there. We want our who clause to add information, not specify when the rest of the sentence is true, so we should add commas:
Mrs Brown, who is standing over there, is my aunt.
An example of the other usage is as follows:
Students who do not listen to the teacher scored poorly on the test.
In this sentence, the who clause specifies that only some of the students scored poorly.
As a general rule, if the sentences both make sense without the other, you should set off the who/which with commas ("Tet is a festival. Tet occurs in late January and early February" means exactly the same as what we said). If they don't, you should probably not have commas ("Students do not listen to the teacher. Students scored poorly." has a different meaning than what we said).