HMS Vanguard's usefulness in 1944 and after the war

Would HMS Vanguard have had little to do if she entered service in 1944?

No. She would still have been useful in 1944 and could counter Soviet Sverdlov-class cruisers after the war until aircraft such as the Buccaneer entered service.

Vanguard would still have been useful if she had entered service in 1944. After the war, Soviet construction of large cruisers provided a role for her as a surface opponent. That role lasted until aircraft such as the Buccaneer could take over the task.