Actually, i haven't missed a single unvaulting, and so i can say that for the last year DE have been pretty consistent with the them, Mirage is the first prime that's been unvaulted twice (i mean before the rest of the frames have been unvaulted).
Only primes whose relics haven't been in rotation from the day that i've started playing are:
Hydroid (haven't been unvaulted longer than Mirage, but have been given out via twitch, so rip tentacle boi);
Volt (Baro brought him recently);
I've also heard that they've already increased amount of unvaultings per year compared to