This is a report of our experience this morning, for the benefit of future inquirers:
We had no difficulty booking by telephone from the US, at (01) 39 02 9280 0360. The initial recorded greeting is in Italian, followed immediately by a prompt in English to press "2" for service in English and other languages. The time on hold was less than 5 minutes. The order-taker was fluent in almost-unaccented English. For our selected date in mid-September, there were many open slots throughout the day -- even though the ostensibly official Website for on-line purchase (www.vivaticket.it) showed no September availability whatsoever except for a few days during the first week. When we asked for a 10:00 a.m. slot, the order-taker volunteered that an English-language guided tour was available at 9:30 a.m. for an additional E3.50 (on top of the basic E.6.50 plus E1.50 booking fee). Email confirmation in both Italian and English arrived within seconds of completing the transaction. The whole experience was quick, efficient, and hassle-free.