Like I said, it's odd. It reads as if Toriyama forgot that Vegeta saw Goku use a Kamehameha already.
I'm telling you Dagon. This gotta be the way it's read...
Ok, I got my ass kicked. I'm going to go to my trust Galick Gun. I can't be defeated when I use my ULTIMATE TECHNIQUE!
Hey! He's matching my Galick Gun! This can't be! It's my Ultimate attack and no one matches my ultimate attack! I am the Prince of all Saiyans, how can a lowly Saiyan match me!!! Rage!!!
We are peering in from a hyper specific power scaling perspective not held by characters.
Gohan- He's way too strong, but I'm confident in my SPEED! Oh no!
Piccolo- He won't keep up with my speed, because I trust in my ULTIMATE SPEED! Oh no!
Not everything can be judged from a power scaling perspective. There are irrational thoughts of fighters. Fighters are often irrational, always assuming they are the best.
Vegeta- I am a Super Saiyan! Even though Freeza's ki is stronger, I CANNOT be defeated! I am the WARRIOR OF LEGEND!
This is the best way to make sense of it I believe.
I don't think Vegeta thinks his amp is so strong is can overwhelm a much stronger fighter's amp.