Vietnam, one of the world’s poorest countries, is to begin operating a second national cellular telephone network this week covering 18 of the country’s 53 cities and provinces, industry executives said. They said the new system was wholly-owned by Vietnam Post & Telecommunications Co using equipment supplied by Siemens AG and Motorola Inc. Details of how much the equipment deal is worth were not available. The state-owned network will compete with the existing Vietnam Mobile Services Co system, a joint venture also involving Vietnam Post & Telecommunications and Sweden’s Comvik International Vietnam AB.