I don't think the musicians you cited as examples are exactly one-hit wonders. According to the All Music Guide, Suzanne Vega has recorded 9 albums, Tracy Chapman 8, and Janis Ian 23.

Concerts and tours? Suzanne Vega has scheduled 18 concerts in 11 countries between Feb 24 and April 25, 2002. Tracy Chapman apparently hasn't announced any future tour, but her most recent one included 84 concerts between March 28 and Sept 23, 2000. (After that, I'd want a year off. Wouldn't you?) Janis Ian has scheduled a tour of about 30 concerts between Mar 8 and Aug 8, 2002. (OK, a FEW of these venues look like they might be coffeehouses.)

I don't know how YOU measure success, but those performers are having very active careers. I think there are a lot of ex-rock stars that would be envious. (And so would most folk musicians.)