I think all we can really take away from that is that Vadderung is an oddball. What it does not say, or even really hint one way or the other, is whether he was able to stay relevant with his full Immortal Power intact or whether he had to give it (or something else) up.
hmm that reminds me of the Odin from Norse mythology,
The guy who would disappear for long periods of time as he walked alongside humanity in is quests for wisdom.
some of you guys must know this better than me but Odin didn't really rely on any typical godlike powers, the mythologies we still know about him mostly focus on Odin learning all kinds of knowledge in crazy ways, it seems perfectly in line with that for Odin to give up the god side of his powers to focus on gaining more knowledge/wisdom/magic, whatever, and the series has portrayed him as knowing just about everything.
the point im trying to make is that all that knowledge was his main focus and he most likely is now in a mortal (but still wizard level) body, but he can wield his low amount of power with his godlike cunning and knowledge base so hes only kinda mortal.
Remember bob explaining the insanely complex magic behind demon reach.
the oddball stuff is true because the rest of the old gods would never consider trading in for a mortal body