The rule of not responding to the Qaddish or Qedusha, applies to the entire prayer of the 'Amidah. However, the 'Amidah is considered to end after the first Yihyu LeRason. In other words, after one recites the last Berakha (blessing), one must immediately say, "Yihyu LeRason", etc. The verse of "Yihyu LeRason" is also considered to be part of the 'Amidah.
After one has said that, the 'Amidah is considered to be technically over, even though one has not yet taken three steps back. It says in Ben Ish Hai, that at this point, one may even answer "Amen" to Berakhoth (blessings). This means that even in the middle of Elokai Nesor, one may answer. When we refer to the "first" Yihyu LeRason, this is based on the custom of Sephardim and some Ashkenazim, of saying one Yihyu LeRason before Elokai Nesor and another after.
The Qisur Shulhan 'Arukh of Rabbi Shlomo Ganzfried, 'a"h, states according to the Ashkenazi custom, that one who only says one Yihyu LeRason, should ideally say the Yihyu LeRason and even 'Oseh Shalom, before answering, wherever possible. He adds, however, that it is preferable to say two Yihyu LeRason's and that, in such a case, there is no problem with answering between the two.
(See Ben Ish Hai, first year, Beshallah, Oth 22. Qisur Shulhan 'Arukh [R' Ganzfried], 18:15)
Yihyu Leratzon, Kitzur Shukchan Aruch, R Shlomo Ganzfried, Answering Kaddish and K'dusha during Shmonei Esreh Qaddish, Qedusha, 'Amidah