Well, vinegar isn't the right stuff if it's apple based. Your RAD will be growing organic nasties thereafter.
Distilled or white vinegar is the ideal stuff to use.
If the RAD is well clogged, some people have tried drain-o, toilet bowl cleaner and the like. I can not vouch for any of those myself. For RAD cleaning other than white vinegar, I've used automotive RAD flush products. The best tends to be Prestone Super Flush. The 22oz small bottle is about $5 and some change. Let it sit in the RAD overnight or for a few hours. Drain the flush into a bucket while using a coffee filter. Notice how much gunk is in the filter. If it's bad, re-run the filtered Super Flush through a second time.
When the RAD is flushed, rinse with distilled or RO filtered water.