I can't say how Vaya are currently, but when I was with them years ago, they would delay turning the data off until we were way over our caps, citing 'lag' as the reason. So even though they looked cheap on paper, they cost us a fortune.
Since leaving optus I've gone with Jeenee and been very happy. You can choose with them whether to automatically add extra data when you reach your limit or cut it off.
This feature is needed desperately! I have 5 phones on my account and would love to cap them. You should be able to choose if they go over rather than automatically
Customers have the ability to manage their data usage and receive usage alerts to assist them in not going over their monthly allowance.
You can opt to have the usage alerts for all the services sent to your service if you wish so you're aware of what's going on.
I have that set up and also monitor usage through the APP. But teenage kids don't listen, and when they're over 100klms away some of the time it's hard to take their phone off them, or get them to turn off their data. Would be great to dynamically turn data off/on per handset within my account so I can force them to stop using it when they have hit their limit, without costing me extra when they go over.