Welcome to JustAnswer, my name is XXXXX XXXXX I'm here to help get your problem resolved. Ask follow up questions, thanks.
It is definitly a heater core leak....You can try some different stop leaks but they are usually temporary and can clog up radiator and heater core.....If you live in a warm area you can disconnect heater core hoses going into firewall adn bypass it....But if you need the heater you need to have it replaced. The part is around $150 and its about a 9 hour job.....If you do add stop leak you might want to disconnect a heater core hose and poor stop leak directly into heater core hose. let me know if you have questions, thanks.
From what I've done and seen in the past.......ONLY VW fluid isnt needed as long as replacement fluid says its compatible....I also wouldnt mix fluids.
I think its something you can do.......But if you decide to I'd buy a repair manual from www.alldatadiy.com which will provide step by step instructions. Let me know if you have questions, thanks.
Thanks, XXXXX XXXXX you!