A brief scuffle that you likely have closer than Cell vs Goku.
Chapter: 398 (DBZ 204), P14.4-5
Context: after Goku fights Cell at full power for a bit
Cell: “Excellent, Son Goku! This is it! Battles aren’t interesting unless [the fighters’] true power is close to a certain extent like this.”
Goku: “Yeah…I think so too.”
Chapter: 400 (DBZ 206), P4.2-3
Context: Goku and Cell continue to fight
Kuririn: “Th-they’re both so fast…”
Cell: “You really are good, aren’t you?...Frankly I didn’t think I’d be able to have so much fun.”
Where are you seeing this with Vegeta and Kaioken x2 Goku? Pretty sure you don't believe Vegeta powered up, but take a look at Goku's reaction after Vegeta released his ki.
Goku was so outmatched that he figured it was better to blow up his body than continue fighting with x2. Vegeta was continually taunting Goku and literally said he was disappointed in his power. Cell on the other hand had nothing but praise for Goku. If anything, I think Vegeta should be somewhat above 18,000 to further widen this gap.