When the vault is unlocked, all of the vault contents exist in a decrypted state in the process memory. In rare cases, your master password or PIN can also be found in the process memory.
The bottom line is that you should not use Bitwarden on any device that could potentially be infected by malware, and you should never leave a device with an unlocked Bitwarden instance unattended.
However, none of this has anything to do with the fact that Bitwarden stores a local copy of your vault on the device. The locally cached vault copy is stored in an encrypted state.