You need to convert the credit to a data pack here: Topping up Mobile Broadband you should be able to queue up to 6 packs. You can find how much data has been used in a single session from your dashboard but this is the device counting, data usage is measured on the network always worth a double check from your online account.
If you still have problems try removing and re adding the device. You can also speak to Vodafone here: Contact Us and the Team will be able to get you connected.
