This is a commercial product, not residential. I don't think Vonage provides any SLA to their customers, nor it is regulated to demand some minimal reliability standards. They are working in "best effort" mode. While it is good for day to day regular phone use, I think it is not suitable for real time alarm system.
"Look! There! Evil!.. pure and simple, total evil from the Eighth Dimension!" -- Buckaroo Banzai