I can say with near certainty that sending sms's while on international roaming will not be included in your cap and will be charged over and above at a rate of $0.75 or higher. Some carriers charge a similar amount for every RECEIVED sms while roaming internationally.
Specifically for Vodafone, outgoing is charged at $0.75:
http://www.vodafone.com.au/personal/...tpay/index.htm