"have received my vrc after purchasing car - how do i now go about taxing it - helpline not being answered"
First Is the car registered in your name or previous owner ? If in your name you should submit a fully completed and signed Motor Tax Renewal Form RF100A (available from Garda Station) to your local tax office e.g Smithfield.
Note: The onus is always on the seller to ensure that the change of ownership notification is completed correctly and submitted without delay to the relevant authority. so I presume the VRC is now in your name.
Rgds
Billo