I think it's either an instance of bad writing like Trunks not realizing his new form was so slow, or the fact that most people that are over 30,000 or whatever are mutants like Ginyu said, so he didn't expect to have bad chemistry with Goku. Either way if toriyama wanted us to think Ginyu believes he can fully control Vegeta's body he'd have said it in my opinion.
Lol fuck off P. Vegeta never makes mention of what he shows against Jheese being his full power, knows Goku can multiply his BP as seen in the Saiyan arc, and Goku himself who was the first to deduce Ginyu would have a hard time controlling his body the same way he did with Ginyu's automatically assumes Vegeta's body would give him the win. Also, Ginyu-Goku only lasted so long against the duo he fought (who still hadn't taken damage) because he had Goku's face, yet they all somehow assume with Vegeta's "weaker" body he will have the upper hand? There's nothing in the narrative to suggest that Vegeta's body will be different than Goku's, whereas Vegeta directly states since ki sense was possible for another Saiyan, it should be so for him as well implying similar body types.
I'm sure there's more holes in your argument, but the shining turd in the windpipe is that Vegeta notes no increase after the nap, and you say he got 9x stronger?
Ok then. I'll just leave it there.