got a go-pro? clamp it under the wheel arch and film it from various angles, you might see something

you can get vibration with a bit of understeer, if the dampers aren't performing then they won't be able to control it. Have you had the dampers out? you might have a failure

It might also be worth borrowing some wheels and tyres off someone else. I have RE003s and they've never vibrated with understeer, but they're fairly new. It may be something that happens to them when they go off