There are several options out there for VAs to sync calendars. If you are not using Microsoft Exchange Server, there are several programs out there you can use. ShareOutlook by 4 Teams is a paid subscription that updates Outlook (appointments, contacts, tasks, etc.) as soon as somone makes a change (depending on how you set up your preferences).
Here's a great article on this topic:
http://www.slipstick.com/outlook/share.asp.