Get yourself a $15 usb endoscope camera that you can hook to your phone, drain the tank, and take a peek inside. That'll give you a good idea of what you're up against.
When it comes to cleaning, honestly, don't bother with vinegar. I highly recommend some Evaporust. It works extremely well and only dissolves rust, and does it way faster than vinegar.
First you should rinse the tank thoroughly with water and degreaser, or acetone, alcohol, mineral spirits or whatever. Then fill with evaporust and let it sit for 24hrs. Drain, rinse with water, then quickly with lots of alcohol to remove the moisture, and then fill with premix.