This is not a Bitwarden issue. Nonetheless, the answer depends on whether or not you care about the contents of the vault_log.ldf file. When you get this warning message, most likely the copy of the vault_log.ldf file contained in the tar file is not in a consistent state. Thus, if you ever need to restore your backup, the vault_log.ldf file may be corrupted and not usable. Would that be an issue for you?
If you need a backup of the vault_log.ldf file, then you will most likely have to create a VSS snapshot. Alternatively, it may be possible to avoid the error if you temporarily shut down the server while performing the backup.