Seu problema é provável devido ao tratamento especial do cron do sinal de porcentagem:
The entire command portion of the line, up to a newline or % character, will be executed by '/bin/sh or by the shell specified in the SHELL variable of the crontab file. Percent-signs (%) in the command, unless escaped with backslash (), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.
Então você precisa escapar deles:
* * * * * tar -czf /backup/$(date +\%F--\%T)-localusers.tgz /vagrant