I know no such thing is ever mentioned, but think about it. On Namek, it was never mentioned that the boost Krillin got from Guru kept giving him strength, but it was obvious from his fights that that was the case.
Likewise, in the anime, though it's not mentioned, we can assume judging from the fights that both Goku and Vegeta absorbed part of Vegetto's power, maybe because the fusion was supposed to be permanent. That explains why they fared well against Gohan and Gotenks in Buu's body and why Goku could fight evenly with Kid Buu, who's stated to be stronger than Buuhan in the anime.
Likewise, in the anime, though it's not mentioned, we can assume judging from the fights that both Goku and Vegeta absorbed part of Vegetto's power, maybe because the fusion was supposed to be permanent. That explains why they fared well against Gohan and Gotenks in Buu's body and why Goku could fight evenly with Kid Buu, who's stated to be stronger than Buuhan in the anime.