Al-Ain (United Arab Emirates) — AFP Sport’s guide to the Asian Cup quarter-finals, which begin on Thursday and will see the video assistant referee (VAR) system used in the tournament for the first time. Vietnam v Japan Japan should brush Vietnam aside but the four-time champions have been so unconvincing that a defeat would not be a major surprise. Hajime Moriyasu’s side have stumbled through the tournament, winning by a single-goal margin in each of their four games and conceding twice in their 3-2 Group F victory over Turkmenistan, ranked 127th in the world. They allowed more than 75 percent possession to Saudi Arabia in a dire last-16 outing that was settled by Takehiro Tomiyasu’s header from a corner against the run of play. Meanwhile Vietnam, fresh from winning the Southeast Asian title last month, beat a well-organised Jordan side on penalties and could prove a handful for the stuttering Japanese. China v Iran The end looks nigh for Marcello Lippi’s China as they run into the tournament’s only consistently dangerous team. Iran are Asia’s top-ranked side and have yet to concede at the tournament, rattling in nine goals their four matches as they moved confidently into the quarters and…