If
this information is for the correct company and is correct, VTL Internet Ltd was struck off the roll in September 2011 and no longer exists. It's possible lines with their tag are now being run by another company - Zen or AAISP may be able to get contact details for you from BT Wholesale.
A further problem with trying to unravel the previous broadband is that you're a third party to that contract and have no right to bring it to an end - though you may be able to demonstrate the existing broadband should be ceased or migrated to you (in which case you'd get a MAC) if you can show the user has left the property.
This is beginning to fall into the "world of pain to sort out" bracket. It's a shame that you'd already entered into a phone contract with a cheap provider, who probably won't be that interested in helping you. They're providing the voice service they're contracted to provide.
I think the only quick and easy ways to sort this are to force a PSTN cease and reprovide on the existing line if you can, or have a second line installed.
Primus may agree to implement a PSTN cease and reprovide if you agree to pay their costs and possibly start a new minimum contract period with them.
Another way to force a PSTN cease and reprovide would be to negotiate early migration away to another provider who agrees in advance to cease and reprovide the line for you if that proves necessary. It may be best to contract with the new provider on a "phone and broadband" basis, so the new provider breaches the contract if they fail to get broadband working for you. Zen may be able to help as the new provider. Though I don't believe Primus can resist early migration away, they may well try to hold you to the charges for the remainder of the minimum contract period. Still, that may be cheaper than having a second line installed and any costs you incur by losing the existing number by ceasing the non-broadband line at the expiry of the minimum contract period with Primus.