Through trade, otherwise you likely won't see him for a while since he was Vaulted earlier this year.
Most likely not.
Primes are unvaulted in order of being vaulted, so if saryn, volt and nekros were vaulted in that order, saryn would be unvaulted first, then saryn, so on and so forth.
the next warframe to be unvaulted is most likely loki prime.
As Martin said, Volt likely will not be coming out anytime soon. The next primes to be Unvaulted will be Loki, Ember, and Frost.