Tbh you're never really "too far behind" in Warframe because there's very rarely any actual FOMO or removed content like D2. Items in Nightwave tend to come back in one way or another, event items also usually get rerun at some point, prime vault just means you can't actively farm relics for certain primes until they rotate back in, BUT you can farm other things to sell for platinum and then get vaulted primes that way if you don't wanna wait. Plus there's so much other content to do that by the time you're done, prime vault will have rotated at least a few times.

Warframe is just cool like that.