Not much stronger. After Vegeta blows up the truck they both starting fighting all out, so I'll use the fight from that point onwards. Also that's a very important line I think most people ignore/don't know about. #18 isn't holding back after the point Vegeta blows up the truck.
#18 headbutts Vegeta, which causes him some pain but does not do any serious damage. That alone would tell me there isn't a big gap between them. #18 manages to take a punch to the gut, she doesn't show any signs of pain, and she doesn't smile when taking the punch, but does smile afterwards.
When it comes to androids and actually showing pain things are a bit complicated. For example the only time either #17 or #18 actually make any sounds of pain is when Cell slams #17 into the floor. Piccolo never actually once makes #17 make any grunts of pain or anything like that, despite Piccolo making plenty himself. So at equal powers, androids apparently have enough pain tolerance to not make any sounds when hit. #17 does make a lot of pained facial expressions, but all of Piccolo's hits are at #17's face, not his stomach. Obviously hitting someone in the face is going to distort their face. #18's losing of her smile when hit could indicate it hurt her a bit. Unfortunately when she finally does get hit in the face t doesn't show her facial expression(mentioned later)
#18 then knees Vegeta in the stomach and smacks him into a mountain. And then Vegeta walks out smiling. Just to clarify, Vegeta got knocked into a mountain by #18, after taking a headbutt and a knee to the stomach when she was going ALL OUT, and smiled.
#18 smacks Vegeta across the face again, sending him towards the mountain, but Vegeta quickly recovers, and manages to headbutt #18 and then smack her down. This tells me that #18 isn't hitting Vegeta so hard that every blow is so powerful he can't recover from it, and then he can hit #18 both:
A) Fast enough so she can't react
B) Hard enough that he can follow it up before she can do anything back.
Unfortunately we don't see #18's facial expressions when Vegeta does both of these hits. It's a shame because it'd let us compare them to the Piccolo vs #17 fight.
Vegeta sends a Ki attack at her. It's not really named which might suggest a generic Ki blast. How strong is a generic Ki blast exactly? Never really stated. A quick, uncharged Ki blast may just be as strong as a punch as a way for a fighter to quickly get a hit in from a long range at the end of the combo. It's definitely not on something like final flash/light grenade level where the fighter specifically relies on charging it up and having the opponent take it head on. In this case Vegeta is simply throwing off a quickly blast to hit #18 before she can recover from his previous combo.
She does show a shocked expression when the blast is coming towards her, but obviously isn't affected by it. But of course due to her infinite stamina, the blast would need to pretty much kill her or it'd have no effect at all. Simply "hurting her" doesn't do anything in the long run.
#18 starts blocking some of Vegeta's hits. Vegeta blocks some of her hits. At this point Tien cheers Vegeta on, indicating that he still has a chance. Trunks thinks to himself that Vegeta is fighting on par with #18. Piccolo does say Vegeta will be killing, shocking Trunks and Krillin (so basically Tien, Trunks and Krillin have all in one way or another thought that Vegeta had a chance at this point), but he only says it's because #18's infinite stamina, and not due to a lack of power on Vegeta's end.
It's at that point #18 starts getting the upperhand, but it's attributed directly to Piccolo's reasoning.
So with all this in mind, we have three separate characters who thought Vegeta had a chance against #18, who was going all out. We have a full health Vegeta walking away from her attacks smiling. And we have Vegeta being able to get some legit hits and blocks in himself.
From the point #18 goes all out, to the point where Piccolo says Vegeta is losing stamina, #18 gets 4 hits on Vegeta, while Vegeta gets 3 on her (4 including the Ki blast) and then they both block each others hits. This seems pretty even until Vegeta loses stamina. Of the 3 hits Vegeta even manages to do one that sends #18 flying, something that can't be done to opponents that can tank your hits. It is worth noting that Vegeta seems to lose stamina much faster than Piccolo did vs #17, which would probably suggest they aren't equal in power.
Despite Trunks saying they're fighting evenly I don't think they have equal PLs. We have an actual fight between an Android and a character at equal PLs and there are some differences. #16 never points out that Vegeta is equal to #18, Vegeta loses his stamina faster and #17 is more visibly hurt by Piccolo's hits than #18 was (though since we don't see #18's face when she's actually hit in her face, I hesitate to use that).
I don't like using numbers but with all this in mind I guess I'd say a gap anywhere between x1.10-1.20 would be okay? If #18 is like #17 then the actual power of her punches probably isn't up to par with her level. Vegeta can fight her on par while at full stamina, despite being weaker through a combination of high speed (training under x300 gravity would make him much faster than most characters at his PL), and better skill.
Edit: Vegeta starting to lose his battle faster than Piccolo could be attributed to Vegeta losing his "state of mind" that Toriyama talked about. Piccolo would likely keep a level head in his fight, while Vegeta would get mad and that'd affect his abilities.
