Araragi did not lose entirely his vampire powers when he sucked Heartunderblade's blood. Just like Shinobu, Araragi became a half existence.
“Araragi-kun, you won’t be able to turn back into a human -- but you
will be able to come very close to it. You will become a
vampire-like pseudo-human. Some of the special characteristics and
abilities of a vampire will remain -- it won’t be possible to strictly
call you a human, but you will be extremely far from a vampire and
extremely close to a human, naturally you will be completely different
from a half vampire, become this kind of halfway entity. It will suit
you.” - Oshino Meme
This can be seen multiple times later chronologically in the series, like when he healed almost instantly his wound when Senjougahara stapled his mouth in the first episode of bakemonogatari.
Even though sucking blood should make him more powerful, he was sucking the blood of his master. Since Araragi's power and existence had become dependent of Heartunderblade in the first place, it was basically a suicide move and Araragi would have died if he had sucked all of Heartunderblade's blood.
But by sucking his master's blood to the utmost limit, Araragi was able to bring Shinobu to the verge of death and bring her powers to a minimum while not making both Shinobu and himself die. Since a servant's power is directly correlated to their master's power, this is why Araragi lost most of his vampire's powers in the process, and so he could not exploit the inherent vampire strength of acquiring power when sucking blood.
“Concretely...... yeah. Araragi-kun, come very close to kill
Heartunderblade. Almost strip her of her special characteristics and
abilities as a vampire -- just make sure she doesn’t die.
Make Heartunderblade more moribund than before. So much that she
won’t even be a shadow of her former self, not even her name will
remain. Make her an inferior existence, a human-like pseudo-vampire
-- an entity that won’t be able to eat humans no matter how hungry she is.” - Oshino Meme
However, if left alone, Shinobu would end up dead since her existence was already on the verge of dying. That is why Araragi has to let Shinobu feed on his blood from time to time: to let her recover just a small part of her power/existence to make sure that she and Araragi don't die.