We have similar diagonal plank subfloor with only felt/hardwood over it downstairs and upstairs with have plank/plywood/hardwood. Upstairs feels much more solid. I personally would put down 1/2 ply the felt then hardwood. Putting down plywood underlayment is easy to do. If your installing hardwood yourself, adding ply makes for a nice smooth surface for the hardwood so is easier during install. Depending on age, you’d be surprised how some subfloor planks will sit a bit lower then another and gaps can form between T&G. Not an issue with ply, just make sure to leave expansion gaps.
EDIT: just saw you are doing LVP, I don’t have much experience with it, but with it being so thin, it think you need a good flat underlayment so it sits flats/imperfections don’t telegraph. Instructions for the LVP should have a list of approved underlayments. Follow what it says.