The primary virtue is to lack vice.
virtus: nominative feminine singular, 3rd declension.
vitio: ablative singular neuter, 2nd declension. It's an "ablative of separation"; that is, ablative after the verb careo.
carere: present active infinitive of the verb careo, carere, carui, cariturus (2nd conjugation). Remember: this verb takes the ablative case ("ablative of separation").
- Magni tumultus eo tempore in urbe erant.
There were great tumults at that time in the city.
tumultus: nominative plural masculine, 4th declension. According to the -us ending, tumultus could be nominative or genitive singular, or nominative or accusative plural. But we know it's nominative plural by (i) the adjective magni (which must agree with tumultus in number, case and gender); and (ii) the verb erant (which reflects the number of the subject).
eo tempore: ablative singular neuter. This is an "ablative of time when".