There are two possibilities in your case. You would need to identify and sort out with the relevant contacts i.e., employer or the plan administrator appointed by the employer.
1st scenario : your company decided to payout all RSUs with a fixed value. This value will be the offer price stated by the takeover company. In this case your employer has to issue a W2 as the settlement was completed in form of CASH. The cash inflow can be from your current employer or the overtaking company (should not matter).
2nd scenario: employer decided to pay you by selling all your shares in market and send cash proceeds to you. These shares can be funded by overtaking company in some cases. In this case, the broker ( may be custodian appointed by the employer ) should issue you 1099-B statement as underlying market transaction was taken place. The broker can be the plan administrator as well.