Colon is a 9 digit random destination tag. An XRP (Ripple) address on Vault has two parts:
<master address> : <destination tag>
The destination tag is very important in order to specify different unique recipients and extremely useful for managing multiple addresses under the same master address.
If you deposit XRP to the Vault master address regardless of the tag, it would all go to one master account and the funds are accessible.
You only get the webhook notification on the specific XRP addresses with specific tags and wont get webhook notifications on any other tags.