It's not that he can't control the 7-string but it's more stiffer to do lazy trem tricks.
You don't think he just prefers using Evo? You know, the guitar he's mainly stuck with for over 15 years? You know, the guitar he wrote this about
"For me, Evo has been the voice of my heart and has seen the depth of my most depressed emotional frames of mind to my most euphoric moments of joy and divine love, and she usually gets the brunt of it all. I have cried, screamed, prayed and bled through that instrument, and like I said, although she is only wire and wood, there is an emotional investment in her."
But of course, he'd give up Evo for a second to play a 7 string if it wasn't more stiffer to do lazy trem tricks....
I feel like others that Vai has great talent but it's wasted on much of what he does for trying to be "way out" & twisted. I just wish he'd compose more than he jams with a whammy! His Flex-able album was a non-ego electric-music recording rather than being just guitar. It shows creativity in song writing, recording techniques and arranging. The Attitude Song is a master class for modern lead guitar where there's no improvised solos - just great composing.
I don't think, a small matter of a quarter of a century after Flex-able, that Steve Vai is "trying" to be anything. He is what he is.