I agree that versatility is better than flexability here.
"We make commercial vehicles versatile" would be okay. It is a bit bland though.
"Making commercial vehicles versatile" would also work as a slogan.
The verb "turn" doesn't work in your version.
I would revise the suggestions in 3 and 4 as follows
Adding versatility to your van
Making your van versatile
and will add another option to the mix: "Bringing versatility to vans"