Recommend a used vintage receiver for under $100?

Craigslist. Technics, Pioneer etc. Google it for user opinions.

Check for NAD or Rotel gear.

Might be able to find a decent Toshiba SA-725/750 for around $100.

Rather than go the vintage route I would suggest something like this. It comes with a warranty and you don't have to worry about bad caps or noisy caps.

Someone just posted a really nice Nakamichi receiver here on Audiogon. I was a Nak dealer for many years and these are incredible sounding units.

http://app.audiogon.com/listings/receivers-nakamichi-ta-2a-1-owner-amazing-stasis-sound-2013-11-15-vintage-equipment-33760

I am not affiliated with this seller in any way. Just wanted to point out a nice piece of gear.

The Dayton listed above is shockingly good.

"Vintage for less than $100"... I see the likelihood of frustration and disappointment with what that budget is going to get you. The "first" repair will probably cost more than the receiver/integrated alone.

I like what Onhwy61 suggested but if she has to have vintage, and that's pretty cool, then good luck.

I gave my grandson one of those Dayton amps listed above. For the money it is a very nice little amp. If you don't need a lot of inputs or features it might be the way to go.