Honestly, having personally practiced voice impressions (one of which was Lanipators Vegeta), luck and speaking from the throat. DBZA's Vegeta is very gutteral in it's execution. It just kinda happens that the slight rasp in Vegeta's voice works it's way into my Old Man Voice. Sadly, I am not associated or affiliated with TFS in any way, shape, or form, and will probably never be lucky enough to be associated with them, but it's always been a dream of mine, so the practice on impressions of their characters ends up slipping into a lot of accents or voices I try to do personally