They need to put in an option play with VJJ. Having to decide to follow VJJ and his 4.3 speed or Fields and his 4.4 speed will put a lot of stress on the D IMO. Right now with Monty and Herbert, those guys don't have the speed to take it to the house.
Speed in the backfield is lowkey a sneaky need for the Bears. Like you said, the stress on the D that Fields AND another 4.3-4.4 runner would put on the defense is crazy. Fields breaks a 50-yarder every week. So, now if you get ends not crashing and LBs/safeties hesitant because of that, you can hit them quick inside for a huge gain.
I'd love the Bears to be in position to sign a guy like Tony Pollard or even Saquon Barkley if their teams let them go. Also, if they can get enough picks in a trade back to get a Jahmyr Gibbs from Bama. Worst case, I'd love them to sign a Jeff Wilson or Mostert from Miami.
Using that much money or a high draft pick on a RB is probably a luxury, but it's a viable option to add something to this offense if you can't add help at WR. Velus is an option in the backfield as well, but Fields, Velus, Mooney, Claypool, Gibbs/Pollard/Mostert all on the field is dynamic and how Miami and SF put pressure on defenses, and they don't even have the QB run factor.