vISDN is a true ISDN/BRI circuit extended to you via your internet access.
Using a one rack space vISDN CPE, you plug in your existing ISDN codec (Telos, Maya, etc) into the CPE and also plug in one (or preferably two) internet connections into the CPE via ethernet. While only a single internet connection is required for core use, with two separate internet connections vISDN is better able to provide low latency and highly reliable connections that ISDN is known for and expected to deliver.
vISDN users are provided with new SPIDs so they can receive inbound calls from the PSTN and, of course, make outbound calls to the PSTN both as ISDN/Data as well as voice.