Hi,
I think, the vouchers are country specific. Have a look at the URL
http://www.sun.com/training/catalog/courses/CX-310-081.xml. It says
Exams purchased on this Web site may only be used in the US. If you reside outside the US, please select a country to inquire about products delivered in your country. Once exam vouchers are purchased you have up to one year from the date of purchase to use it. Each voucher is valid for one exam and may only be used at an Authorized Prometric Testing Center in the country for which it was purchased. Please be aware that exam vouchers are nonrefundable for any reason. It clearly says, each voucher is valid for only one exam and can be used at an Authorized Prometric Testing Center in the
country for which it was purchased. For your sake, it would be better if you call up any prometric center locations in India or Sun office in India and confirm this.
Slot booking is usually 2-3 days in advance, but it also depends on the availability of the slot. So, if you really want to take test on a particulr date, its a good idea to call and book the slot atleast 2 weeks in advance.