The vinegar helps bind it, but soap scum is deposits of fats and oils from the soap, your hair, your skin and what does dish detergent do? Breaks down fats and oils. I used to just wash my shower and bath with plain detergent.

Another trick I found for getting off stubborn build up on glass (in your husbands shower that you've ignored for 2 years but have to clean now you're moving house) is to use a glass or ceramic scraper (a razor blade) flat against the glass with detergent and it slides right off.