You can see right from the very first panel of their fight that Vegeta clearly wasn't expecting #18 to be as strong as she was. He was probably thinking she'd be marginally stronger than #20 at best, but whatever. He's being pushed back by her frantic assault, clearly being overwhelmed, and she's emotionless per usual. When he grabs her arm and tosses her into the rock cliff, she's still completely unfazed, and then she dodges and blocks an attack from him before he gets a single punch in to separate them.
I believe that's when they both admit they've been taking it easy on each other. If that's round 1, #18 definitely took it by having Vegeta overwhelmed for the majority of it apart from two attacks that he managed to land on her, both of which resulted in zilch for his cause. Also, and this may be incredibly minor, look at when they land after that last punch by Vegeta: #18 lands gracefully on her feet while Vegeta is on one knee. Dunno if that means anything, but it's a small detail I caught rereading the chapter.
Round 2 starts with Vegeta's blast to the truck being easily dodged. #18's headbutt sends Vegeta reeling a few feet (very similar to when he let #19 punch him) and leaves him bloodied and ticked off. His punch to her gut bends her over (yeah buddy!), but ultimately results in little reaction from her. On the contrary, when she knees him in the gut, he's wide-eyed and doubled over in pain and shock—very clear difference in their attack strengths there. Vegeta certainly loses this round as well.
Round 3 starts after everyone arrives and Vegeta is in fairly bad shape. He's playing it off because he's Vegeta, but you can see with your own eyes that he's roughed up while #18 is sitting pretty. However, when the fight restarts, while #18 takes the initial lead, Vegeta turns it around quickly by tossing her into the rocks and blasting her with his palm semen stream. When she comes out unscathed, Vegeta moves in and it turns into a melee brawl, which she quickly comes out on top of by breaking his goddamn arm. Trunks tries to interfere and his sword is blocked and broken by #18's forearm (and she doesn't look pissed, dunno where that came from). With everyone else interfering, Vegeta is tossed/slammed into Trunks, which KO's Trunks yet Vegeta manages to play a little possum to attempt to hit #18 with a point-blank blast...which she dodges and punches him down again. He shows extreme defiance and yet against tries to stand, but she knees him in the ribs at least twice and he finally regresses back to Base, KO'd. Round 3 was the closest somehow, yet #18 did the most damage in this round, if that makes sense.
Vegeta vs. #18 Round 1 ~< Vegeta vs. #18 Round 2 < Vegeta vs. #18 Round 3, in terms of how close they appear IMO.
